Initial addition of function data

[B-Teague]

Description

Adding mdn data for functions like rgb(), var(), etc.
Added status, groups, and mdn_url for functions.
Cloned syntax from syntaxes.json file.

Motivation

mdn data did not have status or mdn_url metadata about functions. For example, is target-counter() nonstandard now?

Additional details

I am writing CSS helper functions for CSS-in-JS libraries that are generated based on mdn data and syntax.
I want to provide links to mdn for function syntax and documentation and ignore deprecated nonstandard functions.

css([
   background(rgb(0,0,255)),
   minWidth(px(50)),
   minHeight(px(50))
])

Related issues and pull requests

Relates to #135

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[teoli2003]

Thanks, looks good to me.

Note that, in the mid-term, we plan to retire this repo, though no concrete plans have been made yet.

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