Support retainContextWhenHidden in Custom Editors

[KevinWuWon]

1. Open a tab from an extension using the Custom Editor API (eg. image preview)
2. Switch to another tab
3. Switch back to the custom editor.

Observe that the webview needs to start up again from a blank screen and loses all state.

It would be nice if it were possible to enable retainContextWhenHidden for Custom Editor webview panels.
https://code.visualstudio.com/api/extension-guides/webview#retaincontextwhenhidden

(As mentioned in this comment from the Custom Editors issue)

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Prototyping custom editors (#77789)