feat: make text editor selections setter async in preparation for neovim

[saidelike]

Checklist

• [ ] I have added tests
• [ ] I have updated the docs and cheatsheet
• [ ] I have not broken the cheatsheet

I am making this small pr to be reviewed now. This is to make it easier to merge the neovim branch later.

[saidelike]

I have reverted the selections property as requested.

All tests are passing now (7082 passing (5m)). (I think my previous failure was just due to a local change I hadn't committed.)

This is ready for review. @pokey

[saidelike]

After minor changes. Still 7082 passing (5m) as expected. Ready for review @pokey