Add MR's I'm Reviewing to Sidebar Lists

Problem to Solve

Merge Request Reviewers was added to GitLab. This allows people to be placed in the Reviewers role for Merge Requests. If you're a Reviewer those items won't be surfaced in VS Code.

Proposal

We should add a Merge Requests I'm Reviewing filter to the sidebar in VS Code that retrieves a list of MR's where I'm listed as a Reviewer.

Technical details

• We define default queries (that show in the sidebar) in package.json
• The CustomQuery type is defined in custom_query.ts
• Same way as we use assignee_username we would want to use reviewer_username.

Possible improvements to the existing code

• Use URLSearchParams or similar method to replace the current queryString += '&key=value' mutations that the gitlab_service.ts.fetchIssuables() method uses.
• Increase the test coverage for the same fetchIssuables() method.
  • There are a few happy path integration test scenarios in test/integration/tree_view.test.js
  • We could potentially introduce a Jest uni test because there is a lot of logic branching in the method.