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