Add validation for @external, @requires, @key, restricted default roo…

…t operation names (#25)

* Add validation for @key directives

* Added validation rules for @externals and @requires directives

* Added jest-config and graphql-tag dev dependencies

* exported composition functions

* Validate against invalid usage of root operation type names
Add a validator to warn users against using default root operation
type names when they've specified non-default root operation types.

* Update externals error message

* Add union type handling and update related tests / edge cases.