Add missing syntax highlighter

[iHiD]

We need a Syntax Highlighter for Prolog that works with CodeMirror6.

There is some introductory discussion on this issue: exercism/exercism#5801

[cpmachado]

Please do check SWISH https://www.swi-prolog.org/IDE.html
It's similar, as mentioned in the page. Is it still a requirement?

[iHiD]

https://github.com/SWI-Prolog/swish/blob/24fb445b90bd158de20f19b2cd30ef3ec3f6bf79/web/css/editor.css#L13 etc

(cc @ErikSchierboom @dem4ron)

[ErikSchierboom]

https://github.com/SWI-Prolog/swish/blob/24fb445b90bd158de20f19b2cd30ef3ec3f6bf79/web/js/codemirror/mode/prolog/prolog.js