Return an error if a schema has repeated directives without the repeatable keyword

[pavelnikolov]

It was reported in #502 that this library doesn't enforce uniqueness of non-repeatable directives. This is a bug and such a restriction needs to be enforced unless the repeatable keyword is present in the directive definition.

For example, if we are given two directives:

directive @repeatabledirective repeatable on SCALAR
directive @otherdirective on SCALAR

Then only the first one (i.e. @repeatabledirective) can be repeated multiple times on the same scalar type. If we repeat the @otherdirective more than once, then that should result in an error.

The issue is considered resolved when only directives declared as repeatable can be repeated.

[ostrea]

Good day. I would like to work on it. As far as I understand I need to update this function to return an error in case we see non repeatable directive for a second time. Is it correct?

[pavelnikolov]

@ostrea go for it!