Add pikchr as an alias of Pic

[Alhadis]

Pikchr is a web-centric implementation of the Pic programming language, designed to work with markdown code-blocks instead of Roff documents:

```pikchr
arrow right 200% "Markdown" "Source"
box rad 10px "Markdown" "Formatter" "(markdown.c)" fit
arrow right 200% "HTML+SVG" "Output"
arrow <-> down 70% from last box.s
box same "Pikchr" "Formatter" "(pikchr.c)" fit
```

This pull-request simply adds pikchr as an alias of Pic so that such code-blocks receive syntax highlighting on GitHub.

(Checklist removed as it doesn't apply)