Parse .sls as YAML for nano syntax highlighting

Summary:
As nano added YAML parsing syntax, it's possible to leverage this
to highlight .sls in addition to .yml and .yaml. Even if Jinja2
templates blocks are ignored, it's a great help, especially compared
to no syntax at all.

Fixes T1759.

Test Plan: Deploy on WindRiver

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1759

Differential Revision: https://devcentral.nasqueron.org/D2863