* 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
