Focus notebook find widget when it's shown

- move extensions scanning

* Expose theme name in webviews

Fixes #97663

Adds a data attribute to webviews with the theme name. This is lets extensions write theme specific css for webviews

* Change name to `vscode-theme-name` and add `vscode-theme-kind`

vscode-theme-kind is generic kind: light, dark, high contrast. We already expose this as a classname but for conistency also want to have it as an attribute