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.

modules/gdscript/gdscript_tokenizer.cpp

@@ -744,7 +744,7 @@ void GDScriptTokenizerText::_advance() {
 				}
 				INCPOS(1);
 				is_node_path = true;
-
+				FALLTHROUGH;
 			case '\'':
 			case '"': {