Reuse isHorizontalEdge for RichText

[ellatrix]

Description

This branch removes some duplicate logic in RichText where isHorizontalEdge can be used. We do have to move dom utils though.

How Has This Been Tested?

Make sure backspace can still be used to merge blocks. Also make sure blocks are not merged unless the caret is really at the start or end.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[aduth]

A meta-question which is not so specific to this pull request: What is the long-term plan for utils? It's convenient because it's already exposed as a module, but it concerns me especially because its a single shared scope of variables. The fact that we have named files within it exported by index.js makes me wonder why those individual files aren't themselves modules, e.g. @wordpress/dom. Is it merely the fact that there's overhead in creating a module that deters us from wanting to do this? Or is this too granular? Is utils a legitimate and useful classifier for these types of functions?

[aduth]

A nice consolidation 👍

[ellatrix]

@aduth I was wondering the same thing... Also a lot of these functions in dom.js seem like random utility things rather than some well baked module. Not sure what to do for now.

[ellatrix]

Oh and apart from getScrollContainer (not sure it belongs to that file), these are selection API helper functions. Maybe we should rename that file and/or move it to a separate module.

[youknowriad]

I believe this PR introduced this regression #5345

This probably means isHorizontalEdge works differently than isEndOfEditor

[ellatrix]

@youknowriad It works the same but doesn't seem to account for empty containers.