Webview Api Prototype (#42690)

* 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

src/vs/workbench/api/electron-browser/extensionHost.contribution.ts

@@ -47,6 +47,7 @@ import './mainThreadTelemetry';
 import './mainThreadTerminalService';
 import './mainThreadTreeViews';
 import './mainThreadLogService';
+import './mainThreadWebview';
 import './mainThreadWindow';
 import './mainThreadWorkspace';