Move VS Code extension host to search sidebar webview

Created by: tjkandala

The Sourcegraph extension host is currently loaded in a separate VS Code sidebar webview.

The webview must have been opened at some point to activate, so if a user collapses the extension host webview and restarts VS Code, Sourcegraph extensions will never load. I suspect that this is the problem behind #29299 (closed).

We should roll the extension host and the extension host webview API into the search sidebar webview, which should always activate when a user interacts with the Sourcegraph VS Code extension.