Add command widgets to Textual CLI

[rodrigogiraoserrao]

(This issue is reserved for the EuroPython 2023 sprint. If you are not participating in the sprint, please refrain from working on this issue. Thanks!)

The Textual CLI has many useful commands for developers building applications, like textual keys, textual colors, and textual borders.
This issue concerns itself with adding a subcommand textual widgets to the Textual CLI.

This command should be a terminal version of the current widget gallery.
textual widgets should display a simple but interesting example of each widget together with a link to the reference page for that widget.

Consider having a left menu to jump to the different widgets, akin to how textual borders works.

It is very important that this is easy to maintain.
For example, when a new widget is added to Textual, it should be straightforward to add it to this app.

See #1409 or #1545 for examples on how to add CLI commands.

Good luck and have fun!

[pavelkraleu]

I will look into this issue

[willmcgugan]

Still a good idea. But not a priority. Still happy to consider that linked PR.

[github-actions]

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