Added markdown extensions support

[Plytas]

Description

As proposed in #1294 (comment), this PR adds support for markdown extensions.

One important change - HTML is sanitized before being passed to markdown() function. This still sanitizes user input, but allows markdown to produce "unsafe" HTML like <iframe>.

I'm starting this as draft PR to receive feedback. I'll be able to update docs once desired API is confirmed. Also, let me know if this needs tests and point me to an example I can follow.

Visual changes

No visual changes.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[Plytas]

Ping @danharrin @ryangjchandler.

[danharrin]

I like the idea! Few things:

• I think all the properties and methods should be renamed to commonMarkOptions() and commonMarkExtensions() for clarity
• I think it should apply to the TextColumn and TextEntry, "summarizer" classes too, which can render Markdown using the markdown() method
• Since we're using it in multiple places, extract the properties and methods to a trait CanConfigureCommonMark in Filament\Support\Concerns

[Plytas]

@danharrin I have updated the PR with requested changes.

The failing actions are for Laravel 10. Str::markdown() does not accept extensions as it was introduced in v11.13.0. Is there a good way to check if extension support is available and only then pass them in?

[danharrin]

Laravel 10 will soon be out of security release support, so I will probably drop it in v4 anyway. I will update this PR when I do so.

Thanks for the rest of it, it looks perfect!

[Plytas]

You might have to lock it to at least v11.13.0 then.