With Embroider, we can rely on async `import()` to do the splitting
for us.
This commit extracts from `pretty-text` all the parts that are
meant to be loaded async into a new `discourse-markdown-it` package
that is also a V2 addon (meaning that all files are presumed unused
until they are imported, aka "static").
Mostly I tried to keep the very discourse specific stuff (accessing
site settings and loading plugin features) inside discourse proper,
while the new package aims to have some resembalance of a general
purpose library, a MarkdownIt++ if you will. It is far from perfect
because of how all the "options" stuff work but I think it's a good
start for more refactorings (clearing up the interfaces) to happen
later.
With this, pretty-text and app/lib/text are mostly a kitchen sink
of loosely related text processing utilities.
After the refactor, a lot more code related to setting up the
engine are now loaded lazily, which should be a pretty nice win. I
also noticed that we are currently pulling in the `xss` library at
initial load to power the "sanitize" stuff, but I suspect with a
similar refactoring effort those usages can be removed too. (See
also #23790).
This PR does not attempt to fix the sanitize issue, but I think it
sets things up on the right trajectory for that to happen later.
Co-authored-by: David Taylor <david@taylorhq.com>
