improve font providers

[danielroe]

each provider supports some very interesting things, including axes for variable fonts, that we are not always taking advantage of.

would be nice to:

• support passing options to a provider (e.g. google can accept a text option to refine the subset)
• look at and improve google, fontshare, bunny providers

[qwerzl]

@danielroe I'm wondering how we should provide support for variable fonts. AFAIK some of our providers eg. adobe doesn't support variable fonts, so how should we deal with these providers if default.weight is defined as a string in config? Should we just disable the unsupported providers in such cases?