We merge a faux block with contents from after a split point when
inserting HTML. The merge function presumes this block has already had
fixCursor run on it to ensure we add the <br>s some browsers need to
support correct cursor focusing.
Fixes#451
This is a massive refactor to port Squire to TypeScript, fix a bunch of small
bugs and modernise our tooling. The development was done on an internal
repository, so apologies to anyone following externally for the commit dump;
updates from here should come as real commits again.
Co-authored-by: Joe Woods <woods@fastmailteam.com>