* Webview API prototype 3

Part of #43713

Third try at refining the webview api. This pass reworks  #44165.  Major changes:

- Adds an `id` field to webviews. The id is provided by the extension and identifies the webview. It is used with the new event handling apis.

- Adds a new `onDidChangeActiveEditor` api. This is similar to `onDidChangeActiveTextEditor` but is also fired when you change webviews. It replaces the old `onFocus` and `onBlur` events on the webview itself

- Adds an `onDispose` event ot webviews. This is fired when a webview is closed by the user

- Perist webview state when the editor group changes. This is enabled for all webviews, not just those with keep alive.

* Throw error when trying to access disposed webview

* Improving webview documentation

* Clean up dispose management

* Throw if we receive a bad handle

* Move more event handling to input

* Simplify input updating

* Remove extra container property

* Fixing md security alert button

* Remove extra update container call

* Restore syncing of preview to active editor

* Fixing posting to webview

* Debounce preview updates

* Remove previewUri

* Enable direct window.postMessage instead of window.parent.postMessage

* Fixing scroll position not preserved when updating previews

* Revert parent.postMessage change.

Old behavior was correct

* Properly hide webview container on tab switch

* Make sure we only handle scroll events for the correct document

* Don't try setting negative scroll

* Revert vs code whitespace change

Remove needless check for array in ContextKeyChangeEvent

fixes #44412

fixes #43877

fixes #43877

fixes #43763

Re-enable the unused variables tslint rule for the hygiene task fixes #42157

fixes #44407

Keep item expanded after rename