Restrict `Selector` methods to returning appropriate `Field`'s

[timkendall]

We have the power to restrict what types of Field's are returned (i.e "selected") from a Selector object's method.

Some possible implementation options:

• Inline define union of field names (as shown here)
• Define a union of the entire selection available to a type (ex. type UserSelection = Array<Field<'id', [], string> | Field<'friends', [], any>>>) (this might be the key to getting nullable / list type support)

• Note: This is also a critical piece for supporting "native" fragments

[timkendall]

TypeScript 4.7 add's support for constraints on infer'd types which may make this cleaner to implement.