feat: fetch collaborators when in an organization

[PeterCardenas]

Problem

For organizations or repos you are a collaborator on, the mentions completions doesn't include all the collaborators although the github allows for querying collaborators in these cases.

Solution

Fetch the collaborators instead of the contributors when the user has access to query the collaborators.

closes #1

[petertriho]

Awesome, this makes sense. Thank you!