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/analyzer/errors/match_guard_invalid_expression.gd

@@ -0,0 +1,4 @@
+func test():
+	match 0:
+		_ when a == 0:
+			print("a does not exist")

modules/gdscript/tests/scripts/analyzer/errors/match_guard_invalid_expression.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Identifier "a" not declared in the current scope.