Draft: Implement command widgets to textual CLI

[pavelkraleu]

This PR closes this issue.

textual --help
Usage: textual [OPTIONS] COMMAND [ARGS]...

Options:
  --version  Show the version and exit.
  --help     Show this message and exit.

Commands:
  borders   Explore the border styles available in Textual.
  colors    Explore the design system.
  console   Run the Textual Devtools console.
  diagnose  Print information about the Textual environment.
  easing    Explore the animation easing functions available in Textual.
  keys      Show key events.
  run       Run a Textual app.
  widgets   Explore possible example_widgets.

Example:

textual widgets

[pavelkraleu]

@rodrigogiraoserrao This PR was rushed during the EuroPython sprint. I'm changing it to draft so I can finish it properly.

[rodrigogiraoserrao]

Feel free to finish the PR as far as the functionality goes, with all the widgets, and remove the test you added because it is not the correct way of testing a Textual app. Instead, the test will end up in Textual as a snapshot test, as you can see here:

https://github.com/Textualize/textual/blob/dda2cb2be2f1168716b3b16f56406b304b60b708/tests/snapshot_tests/test_snapshots.py#L592-L605