Address GraphQL string literals with characters outside BMP

[zombiezen]

In 21755d8, I added a TODO for stringification on the test case of scalars with characters outside the Unicode Basic Multilingual Plane.

graphql-server/graphql/value_test.go

Lines 62 to 68 in 2f4f8d7

	{
	// TODO(someday): This behavior is not fully specified.
	// See https://github.com/graphql/graphql-spec/issues/214
	name: "String/OutsideBMP",
	value: testStringValue("\U00010000"),
	want: `"\ud800\udc00"`,
	},

This isn't specified upstream, see graphql/graphql-spec#214. There are likely issues with parsing such characters as well, so I should add more tests on the parser.