Add s.locale() method on schemas

[freekh]

We want to add a s.locale() schema method.
The type of the locale is a subtype of string: s.locale("en_us", "fr_fr") // type: "en_us" | "fr_fr"

NOTE: for optional locales, we're not sure yet. Perhaps: s.locale("en_us").optional("fr_fr"). The type for an optional would be the same as if the locale was not.

The intended way to use it will be:

1. as keys on s.record() (see Extend s.record to support key validation #384)
2. as properties on s.object() (see Add .locale() method on s.object #386)

[freekh]

Alternative for optional is to use objects:

s.locale("en_us", { locale: "fr_fr", optional: true})

This means we can do more things, such as:

s.locale({ locale: "en_us",  value: "whatever", label: "Land of the free", flag: "US"  })

There's some questions here such as what happens if you use multiple labels in different places in the code (we think that's fine: we just display all of them or display an error)... Probably more questions as well...

[freekh]

Other nice things to be able to do would be to add an exclude method, so you could have 1 default definition of locale somewhere in code. Then if you need to exclude certain locales for some reason, you could do that. Maybe include would be nice as well