Adds a proposed webiew serialization api that allows webviews to be restored automatically when vscode restarts

Make it more clear that this command only reveals an existing webview

Fixes #44580

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

Fixes #44597

Moves the log level API to env

* Move the logDirectory to extensionContext

Moves the proposed `logDirectory` to `ExtensionContext` and make it synchronous

* Use string for path instead of URI

Fixes #45784

Part of #44579

* make API stable

* make deleteCount: number | undefined | null

Rename webviewOptions.keepAlive to retainContextWhenHidden to make it more clear what this option does

Fixes #44430