Add Unicon/Icon

## Language name

Unicon (a superset of Icon, which is also added implicitly)

## URL of example repository

https://github.com/stars/nobodywasishere/lists/icon-unicon

## URL of syntax highlighting grammar

https://github.com/nobodywasishere/unicon-vscode

I also have a WIP fork [here](https://github.com/nobodywasishere/linguist), but the `codemirror_mode` and `codemirror_mime_type` are wrong and there isn't a better one in CodeMirror. I probably need to add support there and swap the mode here before this can be merged.

## Most popular extensions

.icn

## Detected language