[StevenACoffman]

Supersedes #510 (by @stealth-rklopper), which I could not update. Fixes #506.

This carries the original commit from #510 unchanged (authorship preserved), rebased onto
current master, plus one follow-up commit.

What #510 does

The working draft of the GraphQL spec lets a directive definition take directives, and lets a
schema extend a directive with more of them (graphql/graphql-spec#1206):

directive @old(reason: String) @deprecated(reason: "use @new") on FIELD_DEFINITION

extend directive @old @tag(name: "b")

• The parser reads the directives of a directive definition into
  DirectiveDefinition.Directives, and the location DIRECTIVE_DEFINITION.
• The parser reads a directive extension into SchemaDocument.DirectiveExtensions.
• ValidateSchemaDocument merges each extension's directives into its directive, and refuses an
  extension of an undefined directive.
• The validator checks a directive's directives at DIRECTIVE_DEFINITION. A non-repeatable
  directive may appear once across a directive and its extensions, as graphql-js 17's
  UniqueDirectivesPerLocationRule checks.
• The formatter writes both forms.

The grammar matches graphql-js 17 (parseDirectiveDefinition / parseDirectiveExtension).

Follow-up commit

Limit the prelude change to the DIRECTIVE_DEFINITION location. #510 also added
graphql-js 17's directive-deprecation introspection to the prelude (__Directive.isDeprecated,
__Directive.deprecationReason, __Schema.directives(includeDeprecated:)). Those break gqlgen:
with gqlgen v0.17.95, a minimal project fails to build because introspection.Directive has no
such fields and Schema.Directives() takes no argument:

graph/generated.go:604:38: cannot call pointer method IsDeprecated on __DirectiveResolver
graph/generated.go:627:38: cannot call pointer method DeprecationReason on __DirectiveResolver
graph/prelude.resolvers.go:28:6: __DirectiveResolver redeclared in this block

With this PR the same project generates and builds. The introspection fields should land
together with gqlgen support, in a separate change.

The prelude is generated by validator/imported/export.js from graphql-js v16.13.2, which has
neither the location nor the @deprecated location. export.js now applies both through
applyPreludeDeviations, so regenerating the prelude keeps them, and fails loudly if a
graphql-js upgrade changes the text they patch.

Also:

• Update the singleLocation doc on validateDirectives, which said merged
  definition-plus-extension lists skip the non-repeatable check; directive definitions are now
  the exception.
• Add tests for arguments on a directive extension (rejected), extending a builtin directive,
  and formatting a comment before an extension.

The builtin baselines of the formatter change with the prelude.