Add a "dev mode" setting

[alexr00]

When I'm working on the extension, I might reload dozens of times in an hour. This can cause me to blow through my rate limit. We should have a githubPullRequests.devMode setting, which limits some of the more expensive API calls that can become problematic:

• Don't Copilot PR status automatically and on a timer
• Collapse all PR queries
• Don't fetch issues

[alexr00]

Marking as verified as this is basically just for me.