- tv's default channel (when lauching `tv`) is now configurable via the
`default_channel` configuration option
- add `RUST_BACKTRACE=1` and `--nocapture` to ci testing for better
debugging
- remove all builtin channels and associated glue code as well as the
`ToCliChannel` and `ToUnitChannel` derive macros
- recode all builtin channels using shell commands (along with `fd`,
`bat`, and `rg`)
- add support for interactive shell commands inside cable channels
- drop the `send_to_channel` feature until further notice (will be
reimplemented later on in a more generic and customizable way)
