Fix -Wimplicit-fallthrough warnings from GCC 8

Adds `FALLTHROUGH` macro to specify when a fallthrough is intentional.
Can be replaced by `[[fallthrough]]` if/when we switch to C++17.

The warning is now enabled by default for GCC on `extra` warnings level
(part of GCC's `-Wextra`). It's not enabled in Clang's `-Wextra` yet,
but we could enable it manually once we switch to C++11. There's no
equivalent feature in MSVC for now.

Fixes #26135.

main/tests/test_gdscript.cpp

@@ -127,6 +127,7 @@ static String _parser_expr(const GDScriptParser::Node *p_expr) {
 
 				case GDScriptParser::OperatorNode::OP_PARENT_CALL:
 					txt += ".";
+					FALLTHROUGH;
 				case GDScriptParser::OperatorNode::OP_CALL: {
 
 					ERR_FAIL_COND_V(c_node->arguments.size() < 1, "");