Add New Slice Grammar to Linguist

[InsertCreativityHere]

Syntax highlighting in GitHub is done through the https://github.com/github-linguist/linguist repository.
Since it would be nice to have our new .slice files highlighted on Github we should open a PR to add this.

However, Linguist (reasonably) only allows adding new file extensions after they reach a certain level of use on GitHub.
As of opening this issue, due it it's newness, .slice sees virtually no usage on Github, so there's nothing to do for now.

When some time has passed, and the new Slice syntax has reached some level of adoption, we should open a PR with Linguist.

[InsertCreativityHere]

See the "What are the requirements to associate a new extension to a language?" section of
github-linguist/linguist#4263