Support repeatable directives

[spawnia]

Recently, the GraphQL Spec officially added support for repeatable directives. Since this allows us to depend on the fact that defining multiple directives is generally allowed, we can start using that in Lighthouse.

There are some features that would benefit from having this available:

• Multiple @where directives on a single field #831
• Eager-load multiple relationships through @with #1132

The current parser already allows to read multiple directives, so we should be able to produce a working implementation. There will have to be some fundamental changes to how we deal with directives.

A proper solution will require incorporating the actual repeatable keyword, including the validation steps that come with it. That part will have to wait on graphql-php supporting it: webonyx/graphql-php#608