…urn (#3066)

This adds:

* ConstValueNode - A subtype of ValueNode which recursively excludes Variables
* Improved syntax error when encountering variable in a const value
* parseConstValue(): ConstValueNode
* isConstValue(): ConstValueNode
* Various refinements to AST types to use ConstValueNode (or a type which includes it) to better align to the spec.

In preparation to TS convertion

Defines a central `GraphQLInputValueConfig` and `GraphQLInputValue` as well as single definitions for converting between them, unifying this common functionality between input values and arguments.

This is a pre-req for #3049

…rtSchema

Fixes #3051

* Adds `valueToLiteral()` which takes an external value and translates it to a literal, allowing for custom scalars to define this behavior.
* Adds `literalToValue()` which does the same in reverse, also allowing custom scalars to define it.
* Deprecates `valueFromASTUntyped()` in favor of `literalToValue()`, replacing all use of it in the codebase and adding a printed warning (via a new `deprecationWarning` method).

This also adds important changes to Input Coercion, especially for custom scalars:

* The value provided to `parseLiteral` is now `ConstValueNode` and the second `variables` argument has been removed. For all built-in scalars this has no effect, but any custom scalars which use complex literals no longer need to do variable reconciliation manually (in fact most do not -- this has been an easy subtle bug to miss).

  This behavior is possible with the addition of `replaceASTVariables`
* The `parseLiteral` function is no longer filled with a default implementation. Callsites need to check for it before calling it. This untangles what would otherwise be a circular dependency. Both callsites are updated.