Bug with unescaped value interpolated into a selector query

```
SyntaxError: Failed to execute 'querySelector' on 'Element': '[hotkey="""]' is not a valid selector.
```

https://github.com/github/markdown-toolbar-element/blob/c3630f64cddd29a98bb8b03965e9baa83f585dc4/index.js#L230

Read index.js around line 230 and reproduce the reported querySelector failure using a hotkey value containing a quote. Trace how that value is interpolated into the selector; the issue is done when the reported value no longer produces an invalid selector and the behavior is verified.

Written by the indexing model from the issue text.