SPDX in FEX's sources
FEX needs to slap SPDX in to all of its sources to make mixed license files less of an issue.

This should just be putting `// SPDX-License-Identifier: MIT` on the first line of every FEX file.
Probably want to make sure it doesn't conflict with with the `Scripts/doc_outline_generator.py` thing that is run every release.

Let's start a discussion about doing this to see if anyone has any comments about this.
- Should it just be `// SPDX-License-Identifier: MIT`  ?
- Any concerns?

I personally have never used SPDX so I don't know if there are any particular problems that could crop up.