Add rule to prevent usage of `Object` type annotation

[vitorbal]

We almost never want to use Object, and should use {} instead. See also the official docs

This would prevent bugs like this one: https://github.com/zapier/zapier/issues/18827#issuecomment-406423982

I think we can use this eslint-plugin-flow rule

[stevemolitor]

I assume the rule will allow the $FlowFixMe type?

[vitorbal]

yeah, $FlowFixMe would still work @stevemolitor! This is only to make sure devs aren't using Object and Function mistakenly!