Use Fira Mono font for code

There has been some discussion on the topic of font changes on the linked parent issue. Thomas pointed out that we make some fairly advanced use of TeX features, and font changes may not be so easy. It seems like this wouldn't apply to our code font at least because monospace rendering is simple.

The rendering of ^^ in reflection code is why I think it's worth revisiting this issue.

There was an example standard PDF rendered with different fonts.

Issue with current code font

In general, our fonts are a bit antiquated and could use improvement. The regular serif font is very thin, and our carets look extremely squashed, which is particularly noticeable in reflection code:

Actually, there are many glyphs our current code font that don't look good. Our & looks like there's way too much detail crammed into a box, and it's somehow less tall than %, which seems to fill the whole box.

I know that it's subjective, but our %:%: and ^^ look extremely ugly to me.

Fira Mono

The proposed Fira Mono is an improvement in my opinion:

Start by reviewing the linked parent issue and the example standard PDF comparing the current font with Fira Mono. Check how the draft renders code glyphs such as ^^, %:%:, and &, and consider the work complete when the code font change is applied without disrupting the draft's TeX rendering.