GDScript: Implement pattern guards for match statement

Within a match statement, it is now possible to add guards in each
branch:

	var a = 0
	match a:
		0 when false: print("does not run")
		0 when true: print("but this does")

This allows more complex logic for deciding which branch to take.

modules/gdscript/tests/scripts/parser/features/allowed_keywords_as_identifiers.gd

@@ -14,3 +14,7 @@ func test():
 
 	var TAU = "TAU"
 	print(TAU)
+
+	# New keyword for pattern guards.
+	var when = "when"
+	print(when)

modules/gdscript/tests/scripts/parser/features/allowed_keywords_as_identifiers.out

@@ -4,3 +4,4 @@ PI
 INF
 NAN
 TAU
+when