GDScript: Allow variables in match patterns

To restore an ability available in 3.x and reduce compatibility changes.

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

@@ -0,0 +1,22 @@
+func test():
+	var a = 1
+	match 1:
+		a:
+			print("reach 1")
+
+	var dict = { b = 2 }
+	match 2:
+		dict.b:
+			print("reach 2")
+
+	var nested_dict = {
+		sub = { c = 3 }
+	}
+	match 3:
+		nested_dict.sub.c:
+			print("reach 3")
+
+	var sub_pattern = { d = 4 }
+	match [4]:
+		[sub_pattern.d]:
+			print("reach 4")