feat(runtime): add runtime.earlyReturn - #347

Problem

import { runtime } from '@aws-appsync/utils' fails to load, because the package exports no runtime. A resolver using runtime.earlyReturn() cannot run at all.

What this does

Exports runtime with earlyReturn(obj, returnOptions). It validates the return options with AWS's messages, raised as Code errors like other rejected input:

Returning early throws an AppSyncEarlyReturn that carries the value and the target. Which handlers still run (data source, response handler, remaining pipeline functions) depends on where the resolver is, so that decision belongs to the host running the resolver; the library only signals it.

Tests

__tests__/runtime.test.js covers the four shapes above, recorded through EvaluateCode on AWS. The local branch of checkResolverValid now treats an early return the way EvaluateCode reports it, as the evaluation result.

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