Add blob viewer / syntax highlighting for LookML

Problem to solve

LookML files are rendered as plain text (example).

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