Extract base functionality for WebviewViewProviders into an abstract class

[charisk]

This is to help with re-usability but also to make the code more readable since plumbing code etc. get moved out of the concrete class.

This also solves the issue of disposal of event subscriptions at the right time (see #2891).

Checklist

N/A:

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.