Make sure we consistently load webview resources

There are many places in the code that do `import Event, { ...} from '.../event'`.

These files depend on electron APIs such as webview. They should life in electron browser rather than browser

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

Fixes #41887

Fixes #41795

Make sure we use the correct path seps and normalization on windows for the new webview protocol

* Allow loading webview outside of file: origin

**Problem**
Webviews are currently always loaded from a file on the disk. This results in the webview running in the file origin, potentially allowing it to access any file on disk. If a webview fails to sanitize workspace or remote input, untrusted code could potentially access files on the user's system.

**Fix**
Add a new option to serve the webview out of a "data:" uri instead. This prevents access to `file://` resources.

In order to allow webviews to still load resources from disk, add a new protocol called `vscode-core-resource://` that only allows access to resources inside of the vscode directory.

Moves extension pages and our release notes to use this new option. These already are pretty locked down. We cannot move the htmlpreview command to use this option as it would break a huge number of existing extensions, however the new webview API will always have this new option enabled.

* Shorted protocol name

Fixes #20823