Split out from #77131

The current webview editor api is very procedural. This model has some problems when it comes to supporting editing resources, but actually does make a lot of sense for webviews that aren't backed by real file system resources. For example, if you have a webview that edits some document in the cloud, you should not be required to implement a custom file system provider just to enable basic saving.

This change moves the `onWillSave` and `webviewEditorState` properties back onto `WebviewPanel` instead of keeping them specific to `WebviewEditor`. The save implementation does not fully work yet, as the will require #81521

Fixes #42243

Update Octicons to use new style

Add support for touch events on the suggest widget (Fixes #78984)

Adopt icon font in tree widgets and panels

xterm@4.1.0-beta8

and update documentation for it. Part of #80375