Markdown block: remove module dependency

[simison]

Currently, Markdown block requires the Markdown module to be present, although we don't rely on the module technically.

Markdown block renders Markdown using a frontend library instead of relying on the PHP module.

UI where you need to go to turn on a separate toggle in settings page just to allow a feature showing up in the block picker is not great. From the user perspective, the act of choosing Markdown block from the block picker is de-factor "turning a feature on".

Module toggle itself is still useful for allowing writing Markdown on classic editor pages and the toggle could be hidden for sites using Gutenberg, but that's outside the scope in here.

This PR removes module dependency, making the block always available in the block picker.

Similar question to Tiled gallery's Photon module dependency; #13471

Changes proposed in this Pull Request:

• Remove markdown module check when registering markdown block

Is this a new feature or does it add/remove features to an existing part of Jetpack?

Modify existing.

Testing instructions:

• Have a Jetpack site (e.g. via https://jurassic.ninja/)
• Confirm that Markdown block is available in block editor only when Markdown module is active. You can activate/disable it via "Writing -> Write posts or pages in plain-text Markdown syntax" toggle in Jetpack settings:
  
• In this branch, the block should be available when the module is off (test e.g. via https://jurassic.ninja/create?jetpack-beta&branch=update/markdown-module-dep&wp-debug-log)

Proposed changelog entry for your changes:

• Show Markdown block in the block picker even if the Markdown module is disabled

[jetpackbot]

	Warnings
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[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello simison! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D32834-code before merging this PR. Thank you!

[zinigor]

Seems to work well, thanks for this! Feel free to label this one as Reviewed

[jeherve]

Was there some discussion about it somewhere? While this works well and makes sense, I wonder if this could create some confusion for folks who didn't turn on that feature and would not expect to see it appear in their block editor.

I am not against the idea, I think it's pretty nice, but I just want to be sure we all agree on that approach.

[simison]

Was there some discussion about it somewhere?

No recent new conversations no, this is just what I think makes sense in "fix the flows" context no matter the cons. ;-) I think at this point there are already so many conversations in that we're well aware of all cons/pros (happy to list them all here if you'd prefer) and just need a decision if this is ok direction.

While this works well and makes sense, I wonder if this could create some confusion for folks who didn't turn on that feature and would not expect to see it appear in their block editor.

Valid point! I think that's fine tho because the confusion other way around when feature isn't available without extra toggle is greater, considering how the block picker is fairly contained & hidden place for features to pop-up in. If that makes sense?

[jeherve]

I'm happy to merge this, and since it's got 2 reviews already, I'll do just that.