Font constants crate?

One thing I have noticed in the Python font ecosystem is endless duplication of core constants; various packages have their own lists of:

* Valid language/script/feature tags
* Conversions between usWeightClass/usWidthClass values and fvar values
* Which scripts go through the Indic or USE shapers
* Well-known names for various weight instances
* Unicode range / codepage bit definitions
* Registered axis tags

and so on. Would there be any interest in hosting a crate in this repo with a common set of font-related constants?

Review the issue discussion and the existing fontations repository structure to understand whether a shared constants crate has an agreed scope. Done would require a settled list of supported font constants and an accepted crate design, but the issue does not name files or tests to use.

Written by the indexing model from the issue text.