feat: open status bar MR link in webview

At the moment, opening an MR via the status bar will open it in the browser. This is an attempt to open the MR (and the issue) of the current branch in the webview.

I use the MR button to go to MRs quite often, so because you can’t easily change labels, assignees, or approve/merge—apart from slash commands—this change might add friction because you’d have to click on Open in GitLab to go to the instance.

Related to #154 (closed)

Screenshots

VD: the user is in the VS Code IDE and clicks on the button titled "MR 2" in the status bar. After that, inside VS Code, the webview opens with the merge request page of the MR !2