feat(deps): support for GraphQL v15

[benjie]

Adds support for GraphQL v15; to minimise the schema diffs it also wraps all of our built-in descriptions (to match GraphQL v14's output).

Performance and security impact are dictated by GraphQL v15 vs GraphQL v14 - but ultimately this is the user's choice so is beyond the scope of this PR.

[benjie]

I've added code that wraps all our old descriptions so that the schema diff is reduced there; I'm now going to sort all the schemas that use buildClientSchema which now seems to trigger the schema to be ordered.

[benjie]

I've reviewed everything except packages/graphile-utils/tests/snapshots/ExtendSchemaPlugin-pg.test.js.snap, packages/graphile-utils/tests/snapshots/makeAddPgTableConditionPlugin.test.js.snap and packages/postgraphile-core/tests/integration/schema/snapshots/simplePrint.test.js.snap. Those diffs are too large to sensibly compare given GraphQL v15 reorders the schema (this is an argument for lexicographically sorting the GraphQL schema, but we've chosen not to do this because we want to know where we're adding the fields).