Add possibility to narrow down key space of KVNamespace about workers-types

[vlovich]

CC @koeninger

[koeninger]

The key provided to kv get needs to be a string, not an arbitrary parameterized type K.

If you want to restrict it further than string, I'd suggest wrapping it with your own functions in your own project-specific code.

[kulla]

The key provided to kv get needs to be a string, not an arbitrary parameterized type K.

Good point. In fact I thought that KVNamespace<K=string> means that K must extend string. In my original PR this was in error. I will write KVNamespace<K extends string = string> so that only subtypes of string can be used as K. See this playground example. Thanks for your correction 😄