feat: \boxslash support

[FernandoChu]

What is the previous behavior before this PR?
\boxslash not supported.

What is the new behavior after this PR?
\boxslash is supported.

[edemaine]

I get the warning No character metrics for '⧄' in style 'AMS-Regular' and mode 'math' when trying your branch (via the Netlify link above). This says that the font doesn't have metrics for this symbol. I'm guessing that it means that the font doesn't actually have this character, sadly. (Did you check? https://opentype.js.org/ is a good tool for inspect the font.) Adding it to the font would be required to accept this, but unfortunately this process is a bit involved.

[FernandoChu]

Thanks for the quick review! Honestly I don't know what's going on so I'll just close this PR and make a macro :)