Fixes#44039
Add a new option that lets extensions override which root folders a webview can load local resources from. Defaults to allowing any resource in the workspace
* Webview api
* Cleaning up markdown webview
* Hooking up dispose
* Fix some merge errors
* Use custom context key to show markdown title bar contributions
* Add basic on focus and on blur events
* Fixing find widget in webview
* Hookup _isDisposed
* Use new extension resource scheme to load markdown extension resources
* Documenting keep alive