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.

(cherry picked from commit fc370b3feb)

editor/plugins/script_text_editor.cpp

@@ -997,7 +997,7 @@ void ScriptTextEditor::_edit_option(int p_op) {
 				tx->set_line_as_breakpoint(line, dobreak);
 				ScriptEditor::get_singleton()->get_debugger()->set_breakpoint(script->get_path(), line + 1, dobreak);
 			}
-		}
+		} break;
 		case DEBUG_GOTO_NEXT_BREAKPOINT: {
 
 			List<int> bpoints;