Font constants crate?

[simoncozens]

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?

[rsheeter]

Makes sense to me. I have questions about whether it should be one crate, several crates, if some of it belongs in font-types, if some of it belongs in harfruzz (what shaper), etc.