BREAKING CHANGE: the format of the cable channel files and more
specifically the preview specification is updated to be a single table
named `preview` (with keys `command`, `delimiter`, and `offset`) instead
of three flat fields.
```toml
preview_command = "echo 3"
preview_delimiter = " "
preview_offset = "{1}"
```
becomes:
```toml
preview.command = "echo 3"
preview.delimiter = " "
preview.offset = "{1}"
```
BREAKING CHANGE: No more builtin previews which means channels currently using `:files:` and other builtins will now need to rely on external tools (examples to come).
- 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)
