Add support for "Open DevTools in Browser" in Sidebar menu (#6736)
This adds a new entry to the DevTools menu in the sidebar to "Open in Browser" (matching the extra option for this when you run the "Open DevTools" command from the VS Code command palette):

This will open whatever the default page is in DevTools for this type of app, and will always open in a browser window regardless of the user-settings for embedded/external.
See https://github.com/flutter/devtools/issues/6563