Add TextServer.get_short_name() and use it in --text-driver

This makes it easier to select TextServers on the command line.
Instead of having to enter the full name in quotes, you can now
just use `advanced`, `fallback` or `dummy` (lowercase).

modules/text_server_fb/text_server_fb.cpp

@@ -111,6 +111,10 @@ String TextServerFallback::_get_name() const {
 #endif
 }
 
+String TextServerFallback::_get_short_name() const {
+	return "fallback";
+}
+
 int64_t TextServerFallback::_get_features() const {
 	int64_t interface_features = FEATURE_SIMPLE_LAYOUT | FEATURE_FONT_BITMAP;
 #ifdef MODULE_FREETYPE_ENABLED