execute: integrate subscriptions and refactor pipeline

[yaacovCR]

execute no longer runs the query algorithm for subscription operations. Rather, subscription operations are performed, as per the spec. subscribe and createSourceEventStream are removed.

See additional comments below.

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[yaacovCR]

Now that #3658 suggests that we may be awaiting a serious refactor, this PR has been reworked to represent a least change approach to the existing API. Rather than list the individual steps to the granularity of every renamed function, I will just list broadly what the PR accomplishes, after which I will list what the PR does not do.

1. updates execute to run the full ExecuteSubscription algorithm rather than just ExecuteSubscriptionEvent, updating the return types of execute and graphql
2. removes subscribe/createSourceEventStream (these would have to be deprecated in a 16.x release).
3. introduces DocumentInfo and ExecutionInfo interfaces that advanced clients can use to analyze a document and set up execution parameters. Internal errors variable -- the only necessarily private information required during execution -- is left within ExecutionContext. This is generally similar to the setup within defer/stream with payload context objects, so the separate variable is not much of an issue (in my opinion).
4. export helpers buildDocumentInfo, coerceVariableValues, as well as buildExecutionInfo so that advanced clients can have some help creating parts or all of the DocumentInfo/ExecutionInfo.
5. expose helpers executeQuery, executeMutation, executeSubscription, and createSourceEventStream which take ExecutionInfo rather than ExecutionContext objects. These functions return appropriately typed versions, i.e. executeQuery/executeMutation do not ever return streams (until defer/stream) merged.
6. no need anymore for the newly introduced *Impl functions within execute

What this PR does not do:

1. compile schemas/documents/field collections/variables/arguments
2. provide runtime or type checks for the arguments to executeQuery, etc. I.e. executeQuery can only take an ExecutionInfo object with an operation that is of type QUERY (for ExecuteQuery) or SUBSCRIPTION (for ExecuteSubscriptionEvent). executeMutation should only work for info objects containing a MUTATION, and executeSubscription and createSourceEventStream should only work for info objects containing a SUBSCRIPTION. That is because that TS does not (yet?) support discriminated unions via nested properties and so it's not as easy to do without introducing repetitive type guards that drill down into the ExecutionInfo object or an additional somewhat unnecessary property on ExecutionInfo itself. Considering the type safety comes at a runtime expense, I am forgoing this for now.

[yaacovCR]

The new exported helpers have not yet been exported from main, and are only available via deep imports. That can/should change once the API is finalized.

[yaacovCR]

Ah => I forgot to mention that I have also snuck in here a new argument to allow total customization of the executor used by EXECUTE_SUBSCRIPTION_EVENT.

As well as a default executor:

export const defaultSubscriptionEventExecutor = (
  exeInfo: ExecutionInfo,
  payload: unknown,
): PromiseOrValue<ExecutionResult> =>
  executeQuery({
    ...exeInfo,
    rootValue: payload,
  });

This workflow allows the buildPerEventExecutionContext function to be removed.