Add support for repeatable directives

[maartenvanvliet]

Adds support for repeatable directives as defined by the spec (https://spec.graphql.org/draft/#sec-Type-System.Directives). See graphql/graphql-spec#472 where the RFC was merged for more information.

This PR does several things:

• adds support for the repeatable keyword for directives in the parser
• directives by default have repeatable set to false
• renders directives with repeatable in SDL
• adds isRepeatable to the directive type available for introspection
• adds repeatable(true|false) to the schema definition macro's placed under directive
• adds validation phase in the document pipeline to check whether a non-repeatable phase is applied multiple times
• changes the directive type to match the spec (other built-in types could also be tightened with non_null declarations to closer match the spec, but can be done in a separate PR)

In theory this could be a breaking change, i.e. if the built-in directives were applied multiple times to the same field in a document this will now trigger an error.

[benwilson512]

@maartenvanvliet this is fantastic, thank you! We will probably target this for the 1.6 release which we hope to go ahead with by the end of the year due to the potential for breakage you note. Will review implementation here shortly.

[binaryseed]

This looks awesome! From my read, it looks like everything is in place & tested.. Nice work!

[binaryseed]

Followup that adds this to the IntrospectionQuery provided by Absinthe: #1008