Add new method modeling panel

[charisk]

Adds a new panel that will be used for modeling a single method at a time.

The new panel is a Webview view, which is different to normal Webviews that we've used so far (it's not a tab, but a panel that you can move around). It's been added to the explorer tab.

In terms of implementation, it re-uses the logic we have around generating HTML for webviews, and follows similar logic in terms of handling messages.

The new view is behind a few feature flag while we flesh out the implementation.

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.

[shati-patel]

😎