Desktop: Fixes #6822: Fix extra lines added to KaTeX source when toggling the rich text editor

[personalizedrefrigerator]

Summary

Updates the markdown-it KaTeX plugin such that it does not add an extra newline to the end of block math.

As I am no longer able to reproduce the code block portion of #6822, this should fix #6822.

Testing

Automated tests are included, however, to test this manually:

1. Create a note with block math. For example,

   Test:
   $$ 3 + 3 $$
   
   More math:
   $$
       \int_0^1 \cos^2 x dx
   $$
   End test.
   

2. Switch to the rich text editor, make a small change

3. Switch back to the markdown editor

4. Switch back to the rich text editor, make a small change

5. Switch back to the markdown editor

Even if the math content changes between steps 1 and 3, it should remain the same between steps 3 and 5.

[laurent22]

Hmm I remember there was some other issue with these new lines which means fixing something would break something else, which is why it was left as it was. But that looks like a good change anyway so let's merge and see if we hear about any regression later on.