This generalizes the "arguments of correct type" and "default values of correct type" to a single rule "values of correct type" which has been re-written to rely on a traversal rather than the utility function `isValidLiteralValue`. To reduce breaking scope, this does not remove that utility even though it's no longer used directly within the library. Since the default values rule included another validation rule that rule was renamed to a more apt "variable default value allowed".
This also includes the original errors from custom scalars in the validation error output, solving the remainder of graphql/graphql-js#821.
ref: graphql/graphql-js#1144
This factors out the enum value validation from scalar value validation and ensures the same try/catch is used in isValidLiteralValue as isValidPHPValue and protecting from errors in valueFromAST.
ref: graphql/graphql-js#1126
This changes the parsing grammar and validation rules to more correctly implement the current state of the GraphQL SDL proposal (facebook/graphql#90)
ref: graphql/graphl-js#1102
* Generalizes building a value from an AST, since "scalar" could be misleading, and supporting variable values within custom scalar literals can be valuable.
* Replaces isNullish with isInvalid since `null` is a meaningful value as a result of literal parsing.
* Provide reasonable default version of 'parseLiteral'
ref: 714ee980aa
ref: https://github.com/graphql/graphql-js/pull/903
# Conflicts:
# src/Utils/BuildSchema.php
# tests/Utils/BuildSchemaTest.php
