Highlight julia-repl code in Markdown specially (#54423)
Fixes #54399 by re-introducing the code seperated out from the styled
Markdown PR at Jameson's request
(https://github.com/JuliaLang/julia/pull/51928#discussion_r1483598716).
The code itself is modelled after [equivalent code in
OhMyREPL](https://github.com/KristofferC/OhMyREPL.jl/blob/b0071f5ee785a81ca1e69a561586ff270b4dc2bb/src/MarkdownHighlighter.jl#L15-L31).
The new `markdown_julia_prompt` face allows people to make the "prompt"
shown in Markdown code visually distinct, to [avoid confusing it with
the REPL prompt at a
glance](https://github.com/KristofferC/OhMyREPL.jl/issues/100). By way
of example, I make it italic by augmenting my `faces.toml` with
```toml
[markdown]
julia_prompt = { italic = true }
```
