GDScript: Allow variables in match patterns

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

modules/gdscript/tests/scripts/analyzer/errors/match_with_subscript.gd

@@ -0,0 +1,5 @@
+func test():
+	var dict = { a = 1 }
+	match 2:
+		dict["a"]:
+			print("not allowed")

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

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Expression in match pattern must be a constant expression, an identifier, or an attribute access ("A.B").

modules/gdscript/tests/scripts/analyzer/errors/match_with_variable_expression.gd

@@ -0,0 +1,5 @@
+func test():
+	var a = 1
+	match 2:
+		a + 2:
+			print("not allowed")

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

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Expression in match pattern must be a constant expression, an identifier, or an attribute access ("A.B").