feat: InferPgEnum utility type

[lbennett-stacki]

A utility type to infer a union type from a pgEnum.

e.g.

const rolesEnum = pgEnum("roles", ["guest", "user", "admin"]);
type Role = InferPgEnum<typeof rolesEnum>

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We can also use infer if preferred but I think this initial contribution is cleanest and most performant

type InferPgEnum<T extends PgEnum<any>> = T extends PgEnum<infer Values> ? Values[number] : never;