Add language options

[moewew]

See #937. Related to #938.

For some uses it would be extremely useful to add language options to the .lbx files. Currently the official way to do this is via separate .lbx files and \DeclareLanguageMapping, but that is simply not viable for a larger number of options as the number of required files would explode.

https://github.com/plk/biblatex/issues/891, https://github.com/plk/biblatex/pull/552, https://github.com/plk/biblatex/issues/555 could benefit from such a feature.

In particular if #938 is implemented we'd want those options to be settable without a language package and maybe overridable (is that a word?) if the language package is loaded with a different modifier setting.