mr list: allow before/after filtering
Problem to solve
glab mr list currently allows various filters to be set, but there does not seem to be a way to limit to before/after a specific date/time.
API - https://docs.gitlab.com/api/merge_requests/ - has created_after, created_before, deployed_after, deployed_before.
While there's a merged_at field, there does not seem to be the corresponding filters in the API. Having filters for the after/before merged times would be great, even if glab implements that on top of the API.
Proposal
Introduce flags for all three after/before times.
Further details
Among the usecases, this would allow answering questions like "how many MRs were merged in December" or "how many MRs were merged in 2025".