Move shell expansion into `--config` lookup (#10219)
## Summary
When users provide configurations via `--config`, we use `shellexpand`
to ensure that we expand signifiers like `~` and environment variables.
In https://github.com/astral-sh/ruff/pull/9599, we modified `--config`
to accept either a path or an arbitrary setting. However, the detection
(to determine whether the value is a path or a setting) was lacking the
`shellexpand` behavior -- it was downstream. So we were always treating
paths like `~/ruff.toml` as values, not paths.
Closes https://github.com/astral-sh/ruff-vscode/issues/413.
