Activate the extension only for .adoc and .asciidoc files.

For other commonly used file extensions (`.asc`, `.ad` and `.txt`), add options in the extension settings.

<img width="430" height="447" alt="Image" src="https://github.com/user-attachments/assets/dd36cc13-0641-414b-8bd4-7a162532dcab" />

cc @mojavelinux 