Consider `--preview` flag for `server` subcommand (#12208)
## Summary
This PR removes the requirement of `--preview` flag to run the `ruff
server` and instead considers it to be an indicator to turn on preview
mode for the linter and the formatter.
resolves: #12161
## Test Plan
Add test cases to assert the `preview` value is updated accordingly.
In an editor context, I used the local `ruff` executable in Neovim with
the `--preview` flag and verified that the preview-only violations are
being highlighted.
Running with:
```lua
require('lspconfig').ruff.setup({
cmd = {
'/Users/dhruv/work/astral/ruff/target/debug/ruff',
'server',
'--preview',
},
})
```
The screenshot shows that `E502` is highlighted with the below config in
`pyproject.toml`:
<img width="877" alt="Screenshot 2024-07-17 at 16 43 09"
src="https://github.com/user-attachments/assets/c7016ef3-55b1-4a14-bbd3-a07b1bcdd323">
