Define "execution result" and "request error result"

[robrichard]

Follow up from #1135

This creates a definition for Execution Result and Request Error Result. The definition of Response is updated to include Execution Result, Request Error Result and Response Stream (for subscriptions).

Incremental Delivery will introduce additional types built on top of this change. I am currently thinking something like this:

type RequestErrorResult = {
  errors: Error[] // request errors only
  extensions?: object
}
type ExecutionResult = {
  data: {} | null
  errors?: Error[] // execution errors only
  extensions?: object
}
type ResponseStream = ExecutionResult[]
type IncrementalStream = [InitialExecutionResult, ...SubsequentExecutionResult];
type Response = RequestErrorResult | ExecutionResult | ResponseStream | IncrementalStream;

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[robrichard]

I moved and adjusted some of the text from the Errors section. Placing it under Execution Result and Request Error Result so we can describe the specific behavior (must be included in Request Error Result, may be included in Execution Result) without the Errors section having conditions for each type.

[robrichard]

We could also define a type Response Result that includes both Execution Result and Request Error Result as subtypes, but adding another layer of hierarchy might be less clear?