Add opt-in GDScript warning for when calling coroutine without await

[mihe]

Resolves godotengine/godot-proposals#12653.

This adds a new MISSING_AWAIT warning to GDScript, which is set to IGNORE by default, and which will be emitted when you try to call a coroutine as a "root" expression in a statement block without using await, meaning a scenario like this:

func _delay(time: float) -> void:
    await get_tree().create_timer(time).timeout

func _ready() -> void:
    _delay(1.0) # Warning: "await" keyword might be desired because the expression is a coroutine.
    print("Surely we must have waited a full second by now, right?")

This is of course a perfectly valid thing to want to do sometimes, in which case you can just disable the warning, like any other:

func _flash_red() -> void:
    modulate = Color.RED
    await get_tree().create_timer(0.2).timeout
    modulate = Color.WHITE

func take_damage(amount: float) -> void:
    health -= amount
    @warning_ignore("missing_await")
    _flash_red()

Note that this does not affect the already existing (and different) error that you get when trying to call coroutines within another expression (e.g. another function call):

func _find_meaning_eventually() -> int:
    await get_tree().process_frame
    return 42

func _ready() -> void:
    @warning_ignore("missing_await")
    print(_find_meaning_eventually()) # Error: Function "_find_meaning_eventually()" is a coroutine, so it must be called with "await".

[vnen]

LGTM

[mihe]

Rebased to resolve conflict.