Syntax Highlight `.mjs`

`.mjs` are not processed for syntax highlighting.

Example:
```
https://addons.thunderbird.net/en-US/thunderbird/files/browse/1041250/file/account-check-ui.mjs#top
https://addons.thunderbird.net/en-US/thunderbird/files/compare/1041250...1033565/file/account-check-ui.mjs#top
```