Add Mojo icon

[christianbator]

Description

Add Mojo icon to related files (.mojo).

Motivation and Context

Working with Mojo files. No syntax package currently exists, but perhaps I'll work on that next for source.mojo.

How Has This Been Tested?

• file_type_mojo.mojo

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.