Updates to schema coordinates (#3044)

[magicmark]

Updates to #3044

Implements the new spec changes:

• Split apart the AST nodes and parsing for the new coordinate types (FieldCoordinate, ArgumentCoordinate etc)
• Added MyEnum::value separator syntax
• Explicit error handling for "containing elements"
• (todo?) throw on returning meta-fields

[martinbonnin]

Thanks for looking into this ❤️

[JoviDeCroock]

Code looks good, mainly feeling conflicted about conflating enum-members and type-fields. Input-fields and type-fields feel similar compared to enum-members

[benjie]

@JoviDeCroock has a good point, and perhaps it would actually be better to represent this in the schema coordinates syntax itself:

Type # Any named type
Type.field # A field (e.g. on an object, interface, or inputObject type)
Type.field(argName:) # An argument (on an object or interface type)
Type::value # an Enum value

:: is the syntax used to reference an enum value in C++, Rust, PHP, Perl and others.

[benjie]

I have raised magicmark/graphql-spec#1; if we agree then I'll also make these changes to this branch and get it merged into #3044

[magicmark]

ack. @benjie done! added the enum changes.

[benjie]

Merging this even though it doesn't pass CI under the instruction of the champion 👍