Implement `type` in GLQL to support filtering work items by their type

type is currently not supported as a token.

Implementing it is tricky, but here's a rough plan that would work:

• MVC 1: Implement for supported issue types; Issue, Incident, Test Case, Requirement, and Task.
• MVC 2: Allow, additionally, Merge Requests but not to be used with issue types.
• MVC 3: After implementing Work Items API support, return work items queries instead of issues for recognized types.
• MVC 4: Add epics as a Work Item type, along with anything else.