Auto-resize `registereditor` splits when their content updates from external register writing

[tuurep]

Related: #2 (issue) and #12 (PR)

Discussed: #11 (comment)

Another idea I had was what if the register changes to be much shorter? Should we shrink it?

For instance, the buffer contains 10 lines and then later it updates (after #12 is in place) to just 1 line. Should we resize the window to be 1 line tall?

Yeah, if it was simple/doable, would like this the most:

• Shrink when contents become smaller through autocmd
• Grow when contents become bigger through autocmd
  • This I actually think is the more important one, it can be pretty jarring when it happens that a oneliner becomes a multiliner and you don't realize it will scroll

But I would anticipate a lot of edge cases so let's see how realistic it is 😄