[ty] Add GitLab output format (#20155)
## Summary
This wires up the GitLab output format moved into `ruff_db` in
https://github.com/astral-sh/ruff/pull/20117 to the ty CLI.
While I was here, I made one unrelated change to the CLI docs. Clap was
rendering the escapes around the `\[default\]` brackets for the `full`
output, so I just switched those to parentheses:
```
--output-format <OUTPUT_FORMAT>
The format to use for printing diagnostic messages
Possible values:
- full: Print diagnostics verbosely, with context and helpful hints \[default\]
- concise: Print diagnostics concisely, one per line
- gitlab: Print diagnostics in the JSON format expected by GitLab Code Quality reports
```
## Test Plan
New CLI test, and a manual test with `--config 'terminal.output-format =
"gitlab"'` to make sure this works as a configuration option too. I also
tried piping the output through jq to make sure it's at least valid JSON
