Simplify crossterm imports

[orhun]

What does this PR do?

Ratatui now re-exports the crossterm crate so that we can use it directly without having to depend on it additionally.

Also, crossterm is now a default feature so I simplified the feature flags as well.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@Narsil 🐻

[Narsil]

LGTM Thanks for this modification !