[4.x] Fix Bard sets being inserted too early

[duncanmcclean]

This pull request attempts to fix Bard sets being inserted too early / in the wrong place.

For example: if you had always_show_set_button set to true and your cursor was positioned at the end of a line, the set would be added just before the last character on that line, rather than after it.

This meant your content looked like this:

This is a sentence

[Bard set]

.

The rest of your content...

I haven't touched Bard / TipTap much but this seems to resolve the issue for me.

Fixes #7858.

[jasonvarga]

When you're inserting into a blank line (like you would if you had the "always show button" disabled) then the blank line appears before the set, which feels a bit odd.

Previously, the set appears exactly where the button was, which feels right. The blank line would appear after the newly inserted set.

[jasonvarga]

Your PR does fix the other issue though. 👍