Add blob viewer / syntax highlighting for LookML
### Problem to solve
LookML files are rendered as plain text ([example](https://gitlab.com/meltano/looker/blob/350dc2ca083d019a09dd997252d92cd162c8ebdb/finance.model.lkml)).
### Proposal
Given that we will be using Looker for gitlab.com (https://gitlab.com/gitlab-org/gitlab-ee/issues/6329), we should at least support syntax highlighting of these files.
### What does success look like, and how can we measure that?
| Before | After |
| --- | --- |
|  |  |
### Links / references
https://docs.looker.com/data-modeling/learning-lookml/what-is-lookml
issue