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/deathknight/DKActions.h

@@ -34,7 +34,7 @@ class CastDeathchillAction : public CastBuffSpellAction
 public:
     CastDeathchillAction(PlayerbotAI* botAI) : CastBuffSpellAction(botAI, "deathchill") {}
 
-    NextAction** getPrerequisites() override;
+    std::vector<NextAction> getPrerequisites() override;
 };
 
 class CastDarkCommandAction : public CastSpellAction
@@ -52,7 +52,7 @@ class CastUnholyMeleeSpellAction : public CastMeleeSpellAction
 public:
     CastUnholyMeleeSpellAction(PlayerbotAI* botAI, std::string const spell) : CastMeleeSpellAction(botAI, spell) {}
 
-    NextAction** getPrerequisites() override;
+    std::vector<NextAction> getPrerequisites() override;
 };
 
 // Frost presence
@@ -61,7 +61,7 @@ class CastFrostMeleeSpellAction : public CastMeleeSpellAction
 public:
     CastFrostMeleeSpellAction(PlayerbotAI* botAI, std::string const spell) : CastMeleeSpellAction(botAI, spell) {}
 
-    NextAction** getPrerequisites() override;
+    std::vector<NextAction> getPrerequisites() override;
 };
 
 // Blood presence
@@ -70,7 +70,7 @@ class CastBloodMeleeSpellAction : public CastMeleeSpellAction
 public:
     CastBloodMeleeSpellAction(PlayerbotAI* botAI, std::string const spell) : CastMeleeSpellAction(botAI, spell) {}
 
-    NextAction** getPrerequisites() override;
+    std::vector<NextAction> getPrerequisites() override;
 };
 
 class CastRuneStrikeAction : public CastMeleeSpellAction
@@ -79,10 +79,6 @@ public:
     CastRuneStrikeAction(PlayerbotAI* botAI) : CastMeleeSpellAction(botAI, "rune strike") {}
 };
 
-// debuff
-//  BEGIN_DEBUFF_ACTION(CastPestilenceAction, "pestilence")
-//  END_SPELL_ACTION()
-
 class CastPestilenceAction : public CastSpellAction
 {
 public:
@@ -90,20 +86,12 @@ public:
     ActionThreatType getThreatType() override { return ActionThreatType::None; }
 };
 
-// debuff
-//  BEGIN_DEBUFF_ACTION(CastHowlingBlastAction, "howling blast")
-//  END_SPELL_ACTION()
-
 class CastHowlingBlastAction : public CastSpellAction
 {
 public:
     CastHowlingBlastAction(PlayerbotAI* ai) : CastSpellAction(ai, "howling blast") {}
 };
 
-// debuff it
-//  BEGIN_DEBUFF_ACTION(CastIcyTouchAction, "icy touch")
-//  END_SPELL_ACTION()
-
 class CastIcyTouchAction : public CastSpellAction
 {
 public:
@@ -126,8 +114,6 @@ class CastPlagueStrikeAction : public CastSpellAction
 public:
     CastPlagueStrikeAction(PlayerbotAI* ai) : CastSpellAction(ai, "plague strike") {}
 };
-// BEGIN_DEBUFF_ACTION(CastPlagueStrikeAction, "plague strike")
-// END_SPELL_ACTION()
 
 class CastPlagueStrikeOnAttackerAction : public CastDebuffSpellOnMeleeAttackerAction
 {