Add keyword filter to !channels command

[alekstu]

Summary

• Adds an optional [keyword] argument to !channels so users can filter the channel list by name instead of paging through hundreds of results
• Filter is case-insensitive substring match, works across both TVheadend and IPTV channels
• Returns a helpful message when no channels match the keyword

Test plan

• !channels with no argument still lists all channels as before
• !channels bbc (or any keyword) returns only channels whose name contains the keyword
• Filter is case-insensitive (!channels BBC and !channels bbc return the same results)
• No match returns no channels found matching **keyword**
• Unit tests pass in CI

[dev-topsoil]

Thanks for the Pull Request!

I like the changes but the unit test are failing, see here and below:

FAILED tests/test_tv.py::test_channels_filter_case_insensitive - AssertionError: assert [] == [{'name': 'Zo...id': 'zorb2'}]
  
  Right contains 2 more items, first extra item: {'name': 'Zorbax One', 'source': 'src-a', 'tvg_id': 'zorb1'}
  
  Full diff:
  + []
  - [
  -     {
  -         'name': 'Zorbax One',
  -         'source': 'src-a',
  -         'tvg_id': 'zorb1',
  -     },
  -     {
  -         'name': 'Zorbax Two',
  -         'source': 'src-a',
  -         'tvg_id': 'zorb2',
  -     },
  - ]
1 failed, 160 passed in 1.99s

If you fix the tests, I'll approve the PR and merge the branch.

[dev-topsoil]

The unit tests passed in CI - the workflow only failed because of repository permissions (guess I should skip posting code coverage as a comment for forked PRs...)

Thanks for the PR. I'll approve and merge it.