VariantParser: Fix reading StringNames with '&'.

Keep support for '@' for now for compatibility.

Fixes #49535.
Fixes #49542.

core/variant/variant_parser.cpp

@@ -190,10 +190,13 @@ Error VariantParser::get_token(Stream *p_stream, Token &r_token, int &line, Stri
 				r_token.type = TK_COLOR;
 				return OK;
 			}
-			case '@': {
+#ifndef DISABLE_DEPRECATED
+			case '@': // Compatibility with 3.x StringNames.
+#endif
+			case '&': { // StringName.
 				cchar = p_stream->get_char();
 				if (cchar != '"') {
-					r_err_str = "Expected '\"' after '@'";
+					r_err_str = "Expected '\"' after '&'";
 					r_token.type = TK_ERROR;
 					return ERR_PARSE_ERROR;
 				}