♻️ String helper class (#24390)

Marlin/src/gcode/gcode.cpp

@@ -1173,10 +1173,7 @@ void GcodeSuite::process_subcommands_now(FSTR_P fgcode) {
   for (;;) {
     PGM_P const delim = strchr_P(pgcode, '\n');       // Get address of next newline
     const size_t len = delim ? delim - pgcode : strlen_P(pgcode); // Get the command length
-    char cmd[len + 1];                                // Allocate a stack buffer
-    strncpy_P(cmd, pgcode, len);                      // Copy the command to the stack
-    cmd[len] = '\0';                                  // End with a nul
-    parser.parse(cmd);                                // Parse the command
+    parser.parse(MString<MAX_CMD_SIZE>().setn_P(pgcode, len));    // Parse the command
     process_parsed_command(true);                     // Process it (no "ok")
     if (!delim) break;                                // Last command?
     pgcode = delim + 1;                               // Get the next command