Add linter rule: no function reassignment

[wgardiner]

Adds linter rule for no function reassignment

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[sebmck]

Thanks for the PR! Everything looks good except for the tests. Use for...of instead of forEach. The usage of forEach here is incorrect since the callback is an async function. It will create a bunch of unhandled promises and it wont be handled by the test callback.

[sebmck]

Thank you!