Add no-unused-expressions to our list of lint rules
Add no-unused-expressions to our list of lint rules

Summary:
See https://eslint.org/docs/rules/no-unused-expressions.
Today I wrote some code that contained '===' but I thought
I had wrote '=' and was confused why things weren't getting
assigned.  This lint rule will solve that issue.

Test Plan: - run ka-lint javascript in webapp

Reviewers: benkraft

Reviewed By: benkraft

Subscribers: csilvers

Differential Revision: https://phabricator.khanacademy.org/D43723

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter