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/druid/DruidActions.cpp

@@ -11,15 +11,15 @@
 #include "AoeValues.h"
 #include "TargetValue.h"
 
-NextAction** CastAbolishPoisonAction::getAlternatives()
+std::vector<NextAction> CastAbolishPoisonAction::getAlternatives()
 {
-    return NextAction::merge(NextAction::array(0, new NextAction("cure poison"), nullptr),
+    return NextAction::merge({ NextAction("cure poison") },
                              CastSpellAction::getPrerequisites());
 }
 
-NextAction** CastAbolishPoisonOnPartyAction::getAlternatives()
+std::vector<NextAction> CastAbolishPoisonOnPartyAction::getAlternatives()
 {
-    return NextAction::merge(NextAction::array(0, new NextAction("cure poison on party"), nullptr),
+    return NextAction::merge({ NextAction("cure poison on party") },
                              CastSpellAction::getPrerequisites());
 }
 
@@ -60,15 +60,15 @@ bool CastStarfallAction::isUseful()
     return true;
 }
 
-NextAction** CastReviveAction::getPrerequisites()
+std::vector<NextAction> CastReviveAction::getPrerequisites()
 {
-    return NextAction::merge(NextAction::array(0, new NextAction("caster form"), nullptr),
+    return NextAction::merge({ NextAction("caster form") },
                              ResurrectPartyMemberAction::getPrerequisites());
 }
 
-NextAction** CastRebirthAction::getPrerequisites()
+std::vector<NextAction> CastRebirthAction::getPrerequisites()
 {
-    return NextAction::merge(NextAction::array(0, new NextAction("caster form"), nullptr),
+    return NextAction::merge({ NextAction("caster form") },
                              ResurrectPartyMemberAction::getPrerequisites());
 }