Move LaTeX related files to subfolder src

[fkz]

The root folder is getting crowded again, so let's bring back some structure.

I wonder if we should use another name instead of src to be more descriptive of the contents, like we also have website and latex-check now 🤔 .

Or alternatively, we could create more smaller repositories for the other tools 🤔.

[Toxaris]

I'm confused about moving fonts and glyphs below src here because these feel like substantial subprojects.