[JS API] 'run a host function' and 'create a host function' don't consistently handle completion records

The algorithm for run a host function
uses ReturnIfAbrupt ([=?=]) when calling the underlying JS function (meaning it can just return the completion record), or otherwise returns the result value(s) converted with ToWebAssemblyValue. But 'create a host function' just treats its result value as a completion record when it asserts that the type is normal or throw and returns result.Value. Maybe 'run a host function' should just convert the JS results to WebAssembly values as it does now and put them back into result.Value, and return the whole completion record.

I noticed this while while working on https://github.com/WebAssembly/exception-handling/pull/301 which changes some of this code. We could just fix the issue there, which would simplify merging the EH proposal into the spec. Or we could fix it in both places.

Read document/js-api/index.bs at the linked run-a-host-function algorithm and compare it with create-a-host-function, then review exception-handling PR #301 for the overlapping change. Done means the completion-record behavior is consistent in both locations and the specification changes integrate with that proposal.