GDScript: Fix and simplify coroutine stack clearing

[dalexeev]

• Fixes Godot crashes when you run a script with a Packed*Array variable and two or more awaits #117049.
• Fixes ERROR: Parameter "_p" is null when running a script with a Dictionary variable and two awaits #117164.
• Regression from GDScript: Fix interrupted coroutines not clearing #116711.

This PR simplifies coroutine stack clearing. When a coroutine resumed, the stack was previously stored in multiple GDScriptFunctionStates (via the Ref<GDScriptFunctionState> first_state; field).

Now, previous GDScriptFunctionStates are no longer held in memory. After a suspended function resumes, responsibility for the stack passes to GDScriptFunction::call(), which cleans it up at the end as usual.

The first_state field was introduced in #19828. I made sure that this change doesn't reintroduce #19823:

extends Node

func _ready():
    while true:
        await get_tree().process_frame
        await get_tree().process_frame
        await get_tree().process_frame
        var a: PackedByteArray
        var b := RefCounted.new()
        print("await 3 idle frames ", Performance.get_monitor(Performance.OBJECT_COUNT))

Before, it was previous_state, introduced in #17291. I made sure that this change doesn't reintroduce #17280:

extends Node

func _ready():
    await test()
    print("ready is over")

func test():
    await $Button.pressed
    await $Button2.pressed

[Repiteo]

Thanks!

[stuartcarnie]

@dalexeev I can confirm it resolves the issues with the MRP in #117339. Thanks!