Remove `TextEmitter` (#20595)
## Summary
Addresses
https://github.com/astral-sh/ruff/pull/20443#discussion_r2381237640 by
factoring out the `match` on the ruff output format in a way that should
be reusable by the formatter.
I didn't think this was going to work at first, but the fact that the
config holds options that apply only to certain output formats works in
our favor here. We can set up a single config for all of the output
formats and then use `try_from` to convert the `OutputFormat` to a
`DiagnosticFormat` later.
## Test Plan
Existing tests, plus a few new ones to make sure relocating the
`SHOW_FIX_SUMMARY` rendering worked, that was untested before. I deleted
a bunch of test code along with the `text` module, but I believe all of
it is now well-covered by the `full` and `concise` tests in `ruff_db`.
I also merged this branch into
https://github.com/astral-sh/ruff/pull/20443 locally and made sure that
the API actually helps. `render_diagnostics` dropped in perfectly and
passed the tests there too.
