Add rule to enforce error codes on flow suppressions

[christianvuerings]

Flow version 0.121.0 allows for more specific error suppressions:
https://medium.com/flow-type/making-flow-error-suppressions-more-specific-280aa4e3c95c

It would be wonderful if we could have a rule which requires these specific error suppressions:

• // $FlowFixMe would fail since it has no valid error code specified
• // $FlowFixMe[incompatible-type] would pass since it does have a valid error code

[Brianzchen]

Flow can enforce this with their configs, include_warnings=true option. Would this lint rule still be necessary? Although flow doesn't highlight warnings in vscode which is a bit annoying.

Thoughts @christianvuerings ?

[christianvuerings]

@Brianzchen We already have include_warnings=true in our config but it would still be great to have the error code enforcement as an ESLint rule as well since:

• We can set it as warning or error in ESLint
• ESLint violations do show up in VSCode