Add support for icons in Mermaid diagrams

[TheManchineel]

May we get support for icons in mermaid in theme=witiko/diagrams@v2 (e.g. logos in architecture diagrams)?

For instance, by passing --iconPacks '@iconify-json/logos' as an argument to mmdc, it is possible to register @iconify-json/logos as an icon set, which enables rendering of diagrams as seen below:

```mermaid
architecture-beta
    group api(logos:aws-lambda)[API]

    service db(logos:aws-aurora)[Database] in api
    service disk1(logos:aws-glacier)[Storage] in api
    service disk2(logos:aws-s3)[Storage] in api
    service server(logos:aws-ec2)[Server] in api

    db:L -- R:server
    disk1:T -- B:server
    disk2:T -- B:db
```

Currently, this gets rendered the same as on GitHub (courtesy of GitHub's mermaid support.

architecture-beta
    group api(logos:aws-lambda)[API]

    service db(logos:aws-aurora)[Database] in api
    service disk1(logos:aws-glacier)[Storage] in api
    service disk2(logos:aws-s3)[Storage] in api
    service server(logos:aws-ec2)[Server] in api

    db:L -- R:server
    disk1:T -- B:server
    disk2:T -- B:db

Loading

While with mmdc it is possible to do this for instance:

mmdc -i input.mmd --iconPacks '@iconify-json/logos'

resulting in:

[TheManchineel]

in general it would be cool to be able to pass additional mmdc parameters upon \usepackage, to be used on this line: https://github.com/Witiko/markdown/blob/b294c35857435f2de407a50b951983f48bdf0d7f/markdown.dtx#L37524

[Witiko]

Thank you for the feature request, @TheManchineel. Seems useful!

in general it would be cool to be able to pass additional mmdc parameters upon \usepackage

The easiest way to implement this feature would be to add an expl3 variable that would expand to mmdc by default but which you could redefine (or append to) to include extra arguments. This is not as convenient as a \usepackage option but there is currently no concept of theme-level options, at least for built-in plain TeX themes like witiko/diagrams@v2, so there is currently no principled way to do that.

[TheManchineel]

A global string variable that can be redefined by the user, such as \mmdcCommand that expands to mmdc by default but can be redefined as mmdc~--iconPacks~'@iconify-json/logos' (or even a seq or tl, sorry I'm not very familiar with expl3 myself) would be awesome! Thanks for the attention on this FR <3

[Witiko]

After #614, you can use different icon packs as follows:

\documentclass{article}
\def\mmdcCommand{mmdc --iconPacks '@iconify-json/logos'}
\usepackage[import=witiko/diagrams@v2]{markdown}
\begin{document}
\begin{markdown}

``` mermaid
architecture-beta
    group api(logos:aws-lambda)[API]

    service db(logos:aws-aurora)[Database] in api
    service disk1(logos:aws-glacier)[Storage] in api
    service disk2(logos:aws-s3)[Storage] in api
    service server(logos:aws-ec2)[Server] in api

    db:L -- R:server
    disk1:T -- B:server
    disk2:T -- B:db 
```

\end{markdown}
\end{document}

Alternatively, you may also use the expl3 API, which allows you to incrementally add different command-line arguments from different parts of your TeX/expl3 code:

\ExplSyntaxOn
\tl_gput_right:Nn
  \g__markdown_diagrams_mmdc_command_tl
  { ~--iconPacks~'@iconify-json/logos' }
\ExplSyntaxOff

This functionality will be part of v3.13.0 of the Markdown package, to be released later today and distributed to CTAN and TeX Live likely sometime before next Tuesday.

[Witiko]

Happy New Year!

[TheManchineel]

Thanks a lot!!! Happy new year. Cheers <3