Fix text/latex MIME type in LaTeXWriter

[odow]

If there's an explicit text/latex MIME, then we should just write that directly. There's no point attempting to round-trip it through the intermediate Markdown representation.

Closes #1401
Closes #1346

This was motivated by jump-dev/JuMP.jl#1881, which is an attempt to build the full JuMP documentation to PDF. There's a lot of math/latex trickery involved, which is why we're running into issues.

[fredrikekre]

Thanks. Instead of duplicating this code, perhaps it can be moved to Utilities.jl as a separate function used in both places? Would be great if you can also add a note in the changelog.

[odow]

It seems like mdparse is only used by the LaTeX and HTML writers for the case of parsing MIME"text/latex", so I applied the fix there instead.

[odow]

Okay I took another pass at this. And after digging through to understand the code a little more, I think it can be simplified to just write out the text/latex directly.

It seems to make PDFs nicely, and it passes tests:
bug.pdf

[odow]

The good news is, with this PR, the entire JuMP documentation builds! Lots of code, lots of examples, lots of plots.

[fredrikekre]

Great, thanks!

[fredrikekre]

JuliaRegistries/General#46954