Refactor getCurrentRichTextSelection to use blockSelection start&end blocks

[atimmer]

Spinning out from #7718.

For getCurrentRichTextSelection, @youknowriad mentioned that we could reuse the block selection already present in blockSelection. This issue tracks that.

[ellatrix]

@atimmer How is this different form #11616?

[atimmer]

It was different back when my PR still contained a global state for RichText selection. I had created two issues to track different issues:

• This one: About refactoring the global state I introduced to be inside the blockSelection.
• Remove local RichText state in favor of global selection state #11616: To drop the local state still present in RichText.

#14640 seems to do both in one fell swoop, so I'll close this issue.

[ellatrix]

Thanks!