GDScript: Add is not operator

[dalexeev]

• Closes Allow usage of is not in GDScript godot-proposals#1319.

func test():
    var s: Variant = "str"
    print(await s is not int)
    print(not(await s is int))
    print(await (s is not int))

Class <unnamed> :
|   Function test(  ) :
|   |   Variable s : Variant
|   |   |   = "str"

|   |   print( (NOTAwait s IS int) )
|   |   print( (NOTAwait s IS int) )
|   |   print( Await (NOTs IS int) )

godot/modules/gdscript/gdscript_parser.h

Lines 1397 to 1402 in b4e2a24

	PREC_TYPE_TEST,
	PREC_AWAIT,
	PREC_CALL,
	PREC_ATTRIBUTE,
	PREC_SUBSCRIPT,
	PREC_PRIMARY,

[Mickeon]

Reminder that @vnen did not even realize this was never a thing.

[adamscott]

Nice and clean. Thanks @dalexeev !

[akien-mga]

Thanks!