Make Executable::run return a JsResult<CompletionRecord>

[jedel1043]

Related discussion on #1541.

Follows from #1542.

For starters we can just always pass a CompletionRecord with a [[value]] equal to the value returned by every node evaluation, as all nodes return a JsValue with few exceptions like Throw.

This feature is being developed in the https://github.com/boa-dev/boa/tree/feature/completion_records branch.

[jedel1043]

Closing in favour of the current implementation. We can revisit this if we reassess that a more spec-compliant design is required.