HorizontalRule component for cross-platform separator block

[koke]

Description

This PR adds a HorizontalRule wrapper to the hr HTML tag, using react-native-hr on the native implementation, to allow for a cross platform separator block.

How has this been tested?

Tested on mobile:

• Create a new post
• Insert a new block, verify that separator is available
• Insert a separator, verify it renders correctly

Tested on web:

• Create a new post
• Insert a separator block, verify it renders correctly, block styles still work
• Preview the post and verify it renders correctly

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[koke]

@gziolo thanks for the suggestions. I'll do some more testing next week and get this ready to review and merge

[koke]

I think this is ready to review. I tested on mobile and web and everything looks ok. I considered adding a README.md to the new primitive, but I couldn't come up with anything good, since it's mostly a wrapper for <hr> with no specific props. If you have any suggestions around documentation, I'd be happy to include that.

[gziolo]

I considered adding a README.md to the new primitive, but I couldn't come up with anything good, since it's mostly a wrapper for <hr> with no specific props. If you have any suggestions around documentation, I'd be happy to include that.

I think this is totally expected to be that simple :)
We need to have very basic docs to ensure they are picked up by Gutenberg Handbook:
https://wordpress.org/gutenberg/handbook/designers-developers/developers/components/

It will be exposed in the sidebar.

[gziolo]

Let's add docs and get it merged 👍