Enforce the use of `let` and `const` by using elsint rules.
When creating the eslint configuration, I had assumed that
`extends: eslint:recommended` would have covered this, but
apparently not!
Existing usage of `var` fixed with `npm run lint -- --fix`.
Fixes: https://github.com/cloudevents/sdk-javascript/issues/97
Signed-off-by: Lance Ball <lball@redhat.com>
Grant Timmerman
Lance Ball
Fabio José