[NEW FEAT] Check spreadsheet file type (Google Sheet or Excel file) and return an error message if the file is latter

[ttsukagoshi]

No description provided.

[ttsukagoshi]

When the spreadsheet URL designated by the user in the add-on interface refers to a .xlsx file instead of a Google Sheets file, the add-on returns a runtime error:

Service Spreadsheets failed while accessing document with id *********.

or in ja,

ドキュメント（*********）にアクセス中に　スプレッドシート のサービスに接続できなくなりました。

It would be better for the user if they can see a more specific error message than this general error. Possibly check the spreadsheet file's extension before proceeding with the mail merge process?