Use `v8Str` to Simplify Code Segments that create `v8::Local<v8::String>`

Instead of writing v8::String::NewFromUtf8(isolate_, "<property name>").ToLocalChecked() each time, we can instead make use of our helpful utility function v8Str(isolate_, "<property name>")

This utility function is easier to read and fits perfectly for such use cases.

Note:

• This ticket has been created as per the discussion in the Gerrit Patch discussion thread: https://review.couchbase.org/c/eventing/+/208433/comment/4910dafb_9bfd10c1/