feat(schema) add StringEnum to typebox module

[marshallswain]

Exports the StringEnum utility copied from the TypeBox docs.

// import the module, first
import { StringEnum, Static } from '@feathersjs/typebox'

const T = StringEnum(['crow', 'dove', 'eagle'])

type T = Static<typeof T>

I also added a TIP to the Enum utility to point people to StringEnum.

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