Add syntax highlighting for Fish

Problem to solve

Fish is cool. We use it always instead of Bash. I want to document commands in the wiki in Fish, but its syntax is not highlighted properly:

set now (date)
echo It is $now

Intended users

Wiki writers that love Fish.

Proposal

I imagine the markdown rendering engine in gitlab should support this language.

Permissions and Security

Shouldn't be affected.

Documentation

I guess it's not needed. Writers just have to write their fish snippets with:

```fish
echo Your fishy code here
```

Testing

Absolutely no idea! 🤓

What does success look like, and how can we measure that?

Success would be that fish snippet highligh properly.

Links / references