fix: accept readonly array in concatJoishiTokens function

Fixed TypeScript compilation error where concatJoishiTokens was expecting
a mutable array but received a readonly array from the tokenize function.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: azu

src/token-utils.ts

@@ -48,7 +48,7 @@ const concatToken = (aToken: KuromojiToken, bToken: KuromojiToken) => {
  * @param {Array} tokens
  * @returns {Array}
  */
-export const concatJoishiTokens = (tokens: KuromojiToken[]) => {
+export const concatJoishiTokens = (tokens: readonly KuromojiToken[]) => {
     const newTokens: KuromojiToken[] = [];
     tokens.forEach((token) => {
         const prevToken = newTokens[newTokens.length - 1];