Add TOML files to `SourceType` (#6929)
## Summary
This PR adds a higher-level enum (`SourceType`) around `PySourceType` to
allow us to use the same detection path to handle TOML files. Right now,
we have ad hoc `is_pyproject_toml` checks littered around, and some
codepaths are omitting that logic altogether (like `add_noqa`). Instead,
we should always be required to check the source type and handle TOML
files as appropriate.
This PR will also help with our pre-commit capabilities. If we add
`toml` to pre-commit (to support `pyproject.toml`), pre-commit will
start to pass _other_ files to Ruff (along with `poetry.lock` and
`Pipfile` -- see
[identify](https://github.com/pre-commit/identify/blob/b59996304fea80025d32abc3ac8f7212a9e7380d/identify/extensions.py#L355)).
By detecting those files and handling those cases, we avoid attempting
to parse them as Python files, which would lead to pre-commit errors.
(We tried to add `toml` to pre-commit here
(https://github.com/astral-sh/ruff-pre-commit/pull/44), but had to
revert here (https://github.com/astral-sh/ruff-pre-commit/pull/45) as it
led to the pre-commit hook attempting to parse `poetry.lock` files as
Python files.)
