Fixes philc#3550

Update `content_scripts/link_hints.js` to make elements with class 'btn' clickable.

* Add logic to `getLocalHintsForElement` to check for class 'btn' and mark elements as clickable.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/philc/vimium/issues/3550?shareId=XXXX-XXXX-XXXX-XXXX).

…redundant check for class 'button'

* Merge the if statement for 'button' and 'btn' classes
* Remove redundant check for class 'button'