autocomplete: Add support for composing "silent mentions".

[ungps]

Add markdown syntax for "silent mentions", which happens when
a user starts their mention with "@_". Add empty "silent" class
for future styling changes.

There is also a PR with some changes on zulip-markdown-parser repo. zulip/zulip-markdown-parser#1

Issue: #3374

[gnprice]

Thanks @ungps !

   const filter: string =
     text.length > lastIndex + 1 && !['\n', ' '].includes(text[lastIndex + 1])
-      ? text.substring(lastIndex + 1, text.length)
+      ? text.substring(
+          lastIndex + 1 + (text[lastIndex] === '@' && text[lastIndex + 1] === '_'),
+          text.length,
+        )
       : '';

Oof, this is pretty complicated. 😩

This code is already too complicated -- see #3345 for discussion. It's already hard to understand, and I think that's hiding some bugs already. It gets significantly more so with this change.

I'm going to declare a moratorium on adding features to this code without making it simpler to compensate. @AB261 is working on that right now in #3390. If you can make small local changes to this spot to keep the complexity down, we can move forward on this independent of #3390; otherwise, let's wait for that work to play out.

Separately:

+.silent {
+}

This has no effect and isn't necessary; so just leave it out.

[gnprice]

Closing, for the reasons above. @ungps , thanks again, and please feel free to send a new PR, now or later, as described here:

If you can make small local changes to this spot to keep the complexity down, we can move forward on this independent of #3390; otherwise, let's wait for that work to play out.