NextAction refactoring to eliminate sentinel arrays and pointers (#1923)

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# Description

This PR aims to refactor the NextAction declaration to achieve two
goals:

## Eliminate C-style sentinel arrays

Currently, a double pointer (`NextAction**`) approach is being used.
This an old pre-C++11 (< 2011) trick before `std::vector<>` became a
thing.
This approach is painful for developers because they constantly need to
declare their `NextAction` arrays as:
```cpp
NextAction::array(0, new NextAction("foo", 1.0f), nullptr)
```
Instead of:
```cpp
{ new NextAction("foo", 1.0f) }
```
The first argument of `NextAction::array` is actually a hack. It is used
to have a named argument so `va_args` can find the remaining arguments.
It is set to 0 everywhere but in fact does nothing. This is very
confusing to people unfamiliar with this antiquated syntax.
The last argument `nullptr` is what we call a sentinel. It's a `nullptr`
because `va_args` is looking for a `nullptr` to stop iterating. It's
also a hack and also leads to confusion.

## Eliminate unnecessary pointers for `NextAction`

Pointers can be used for several reasons, to cite a few:
- Indicate strong, absolute identity.
- Provide strong but transferable ownership (unlike references).
- When a null value is acceptable (`nullptr`).
- When copy is expensive.

`NextAction` meets none of these criteria:
- It has no identity because it is purely behavioural.
- It is never owned by anything as it is passed around and never fetched
from a registry.
- The only situations where it can be `nullptr` are errors that should
in fact throw an `std::invalid_argument` instead.
- They are extremely small objects that embark a single `std::string`
and a single `float`.

Pointers should be avoided when not strictly necessary because they can
quickly lead to undefined behaviour due to unhandled `nullptr`
situations. They also make the syntax heavier due to the necessity to
constantly check for `nullptr`. Finally, they aren't even good for
performance in that situation because shifting a pointer so many times
is likely more expensive than copying such a trivial object.

# End goal

The end goal is to declare `NextAction` arrays this way:
```cpp
{ NextAction("foo", 1.0f) }
```

> [!NOTE]
> Additional note: `NextAction` is nothing but a hacky proxy to an
`Action` constructor. This should eventually be reworked to use handles
instead of strings. This would make copying `NextAction` even cheaper
and remove the need for the extremely heavy stringly typed current
approach. Stringly typed entities are a known anti-pattern so we need to
move on from those.

src/strategy/generic/QuestStrategies.cpp

@@ -14,7 +14,7 @@ void QuestStrategy::InitTriggers(std::vector<TriggerNode*>& triggers)
     PassTroughStrategy::InitTriggers(triggers);
 
     triggers.push_back(
-        new TriggerNode("quest share", NextAction::array(0, new NextAction("accept quest share", relevance), nullptr)));
+        new TriggerNode("quest share", {  NextAction("accept quest share", relevance) }));
 }
 
 void DefaultQuestStrategy::InitTriggers(std::vector<TriggerNode*>& triggers)
@@ -22,11 +22,11 @@ void DefaultQuestStrategy::InitTriggers(std::vector<TriggerNode*>& triggers)
     QuestStrategy::InitTriggers(triggers);
 
     triggers.push_back(new TriggerNode(
-        "use game object", NextAction::array(0, new NextAction("talk to quest giver", relevance), nullptr)));
+        "use game object", {  NextAction("talk to quest giver", relevance) }));
     triggers.push_back(new TriggerNode(
-        "gossip hello", NextAction::array(0, new NextAction("talk to quest giver", relevance), nullptr)));
+        "gossip hello", {  NextAction("talk to quest giver", relevance) }));
     triggers.push_back(new TriggerNode(
-        "complete quest", NextAction::array(0, new NextAction("talk to quest giver", relevance), nullptr)));
+        "complete quest", {  NextAction("talk to quest giver", relevance) }));
 }
 
 DefaultQuestStrategy::DefaultQuestStrategy(PlayerbotAI* botAI) : QuestStrategy(botAI) {}
@@ -36,14 +36,14 @@ void AcceptAllQuestsStrategy::InitTriggers(std::vector<TriggerNode*>& triggers)
     QuestStrategy::InitTriggers(triggers);
 
     triggers.push_back(
-        new TriggerNode("use game object", NextAction::array(0, new NextAction("talk to quest giver", relevance),
-                                                             new NextAction("accept all quests", relevance), nullptr)));
+        new TriggerNode("use game object", {  NextAction("talk to quest giver", relevance),
+                                                             NextAction("accept all quests", relevance) }));
     triggers.push_back(
-        new TriggerNode("gossip hello", NextAction::array(0, new NextAction("talk to quest giver", relevance),
-                                                          new NextAction("accept all quests", relevance), nullptr)));
+        new TriggerNode("gossip hello", {  NextAction("talk to quest giver", relevance),
+                                                          NextAction("accept all quests", relevance) }));
     triggers.push_back(
-        new TriggerNode("complete quest", NextAction::array(0, new NextAction("talk to quest giver", relevance),
-                                                            new NextAction("accept all quests", relevance), nullptr)));
+        new TriggerNode("complete quest", {  NextAction("talk to quest giver", relevance),
+                                                            NextAction("accept all quests", relevance) }));
 }
 
 AcceptAllQuestsStrategy::AcceptAllQuestsStrategy(PlayerbotAI* botAI) : QuestStrategy(botAI) {}