Add Appendix C - Built-in Definitions

[martinbonnin]

As we dive into full schemas, introspection and more (see #1036), I'd find it useful to have an authoritative source for built-in definitions that come with a GraphQL implementation (scalar, directives & introspection types).

It shows the built-in definition at a glance and makes it easy to diff, capture a change by looking at the git history. It's also nice for library authors because it uniformizes the descriptions.

The definitions are taken from graphql-js printIntrospectionSchema() with two small changes:

• I added the scalars definitions manually
• I carried over the __Type.fields, __Type.interface, etc... descriptions from the current spec comments

I was doing this for my own needs so figured out I might as well contribute it to the spec but I'm happy to move it somewhere else if you think this doesn't belong in the spec.

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[martinbonnin]

Would that qualify as an editorial change?

The validation/execution behaviour is essentially unchanged. The main thing that could be considered a behaviour change is that the definitions proposed here contain descriptions. We could either:

• Remove descriptions
• Add language that mandates the implementations use the same description as in the spec
• Add language that the implementations are free to change the description if they want
• Leave as is

[benjie]

Let's keep this to just adding the appendix, it does not need to be referenced in the spec text (just as Appendix B is not referenced when grammar is mentioned).

Suggested changes were adopted.