Highlight `*.mjs` files as JavaScript

Created by: olafurpg

Fixes https://github.com/sourcegraph/sourcegraph/issues/34179

Test plan

Ran Sourcegraph locally and verified that *.mjs files are correctly highlighted.

https://github.com/sourcegraph/sourcegraph/issues/34179

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.

Created by: varungandhi-src

Review: Approved

Merged by: olafurpg at 2022-08-17 10:27:04 UTC