* Split out `Webview` from `WebviewEditor` in API
**Problem**
The current proposed `Webview` interface has a few methods and properties that are very editor specific, such as `.reveal` and `.onDidChangeViewState`. These properies will not make sense if we ever allow webview to be displayed in other locations, such as in widgets
**Proposal**
Split the concepts of a `Webview` and of a `WebveiwEditor`. A webview is the html content itself. A `WebviewEditor` is an editor that displays a `Webview`
This would allow us to easily add other types of `Webview` owning objects in the future without having to document that some methods only apply when a webview is used as an editor vs as a widget
As discussed in #45994, move from using a uri to using a viewType. The view type is shared among all webviews of a given type, such as all markdown previews
Fixes#44575
After discussions, we settled on making the webview private unlike `TextEditors`. This means that webview events will live on the webview object itself
Fixes#44571
