Add support for kroki diagrams

[UFOMelkor]

This might be at least ready for discussion.

It adds a renderer for kroki that directly renders diagrams as external images.

So a diagram like this

```blockdiag
blockdiag {
  Kroki -> generates -> "Block diagrams";
  Kroki -> is -> "very easy!";
  Kroki [color = "greenyellow"];
  "Block diagrams" [color = "pink"];
  "very easy!" [color = "orange"];
}
```

will be rendered as

<img src="https://kroki.io/blockdiag/svg/eNpdzEEKwjAQheF9T_GafU8guui2RxAXYzuEkJgpE1GCeHcZdDG4fLyP_1pkzVuiiNcALCo5YTohcmWlOzcbYTYEU0q3Fg5epi95sHYwtT66-7xKEcURISpz7VyKPMPFwH_T2T3V_EOu6oAo1chG3h-IfzvN"/>

Kroki supports the following formats:

• BlockDiag
• BPMN
• Bytefield
• SeqDiag
• ActDiag
• NwDiag
• PacketDiag
• RackDiag
• C4 with PlantUML
• D2
• DBML
• Ditaa
• ERD
• Excalidraw
• GraphViz
• Mermaid
• Nomnoml
• Pikchr
• PlantUML
• Structurizr
• Svgbob
• TikZ
• Vega
• Vega-Lite
• WaveDrom
• WireViz

Although this renderer could possibly render mermaid diagram, it is added after the mermaid renderer, so the latter one will render this in Zettlr.

I ignored caption and width attributes for now. IMHO this makes more sense to handle after the parsing of pandoc attributes has been done.

A general dark mode is currently not supported by kroki (see yuzutech/kroki#1281). For PlantUML I manually added a rudimentary dark mode.

Resolves #4375

[dustdfg]

It doesn't seem to me a very good idea to use external site. Local tool is better IMO. IIUC you just try to get a generated svg file from kroki.io. Is it right?