Split and merge `syntaxes.json` into `functions.json` and `types.json`

[yarusome]

What information was incorrect, unhelpful, or incomplete?

syntaxes.json contains data for both CSS functional notations and CSS types, but there already exist functions.json and types.json for such data.

I'm working on a script for diffing mdn/data and w3c/webref, and the existence of syntaxes.json would make the script less useful since this file increases the complexity of comparison bewteen the two data sources.

What did you expect to see?

This file be split and merged into functions.json and types.json.

Incidentally, functions.md is not at all explaining the structure of functions.json, so it should be corrected as well.

Do you have any supporting links, references, or citations?

No response

Do you have anything more you want to share?

I'll make a PR if this reorganization is agreed but no one does it, but that would be after I having finished writing and testing the script (probably in June).