feat Add charset for StringBone

[ArneGudermann]

Resolves https://github.com/viur-framework/viur-shop/blob/f6e84e772be01d7cf2beb74d674a3aeda6ac303b/src/viur/shop/skeletons/discount_condition.py#L115

[phorward]

Hi there... I'm a little unhappy with this extension, because it can also be simply resolved using a vfunc-lambda, like this:

blah = StringBone(
    required=True,
    descr="Blah",
    vfunc=lambda value:
        "Not all letters are available in the charset"
        if any(char not in "abcdefg" for char in value)
        else None,
)

We should discuss about this.

[phorward]

This should be solved differently: Provide some helper functions in StringBone which can be inserted as vfuncs.

[phorward]

Substituted by #1183