Support explicit values in flag properties, add C# flags support

[raulsntos]

master version of #59328

• Add support for explicit values in properties using PROPERTY_HINT_FLAGS that works the same way it does for enums.
• Fix enums and flags in VisualScriptEditor (it wasn't considering the explicit value).
• Use PROPERTY_HINT_FLAGS for C# enums with the FlagsAttribute instead of PROPERTY_HINT_ENUM.

This allows specifying the explicit value of flags in GDScript and C#:

@export_flags("Red:1,Green:2,Blue:4,RedAndGreen:3") var color: int;

[Flags]
public enum ColorFlags
{
	Red = 1,
	Green = 2,
	Blue = 4,
	RedAndGreen = Red | Green,
}

public partial class MyClass : Node
{
	[Export]
	public ColorFlags Color { get; set; }
}

Uses the same syntax that enums use.

flags.mp4

Also fixed enums and flags in VisualScriptEditor (something like @export_enum("Something:3,Other:4") wasn't working) and adds support for explicit value flags as it was done for GDScript and C#.

I was unable to test the VisualScriptEditor changes in this PR (4.0) but I could test it in the 3.x version:

[akien-mga]

Thanks!