Add webview restoration api proposal

[mjbvz]

From #45994

Adds an experimental API that allows extensions to persist webviews and restore them after vscode restarts. Uses this api to restore markdown previews.

This is done by:

• Adding a new state field on webviews. This is a json serializable blob of data

• Adds a new WebviewReviver interface (name will probably change). This binds a specific viewType to a provider that can restore a webview's contents from its state

• In VS Code core, persist webview editors. When we restart and need to show a webview, activate all extensions that may have revivers using the onView:viewType activation event.

Current implementation is sort of a mess. Will try to clean things up

[jrieken]

@mjbvz What do you want me to review here? I think we discussed the API and I still think over-eager state synchronisation isn't the way to go but an explicit hook is way better. For the rest - I am not familiar with the code around webview editor et al anymore.

[mjbvz]

@jrieken Please take a look at the new vscode.proposed.d.ts API. A few questions:

• How does the serializer api look?

• Should serializeWebview and deserializeWebview be async?

• Should our activation events make it clear that these are webview activations? The current activation event is: "onView:markdown.preview". We could change this to something like: "onWebbiew:markdown.preview" or "onView:webview.markdown.preview"

[jrieken]

Looks good. Yeah, I think the de/serialise-functions should be async (but maybe time-boxed). We already have the onView-event and for the sake of simplicity I'd stick with that, like onView:markdown.preview