Update `@typescript-eslint` to v3

[SimenB]

If possible without extra peer dep warnings, updating to eslint@7 would be nice as well

[SimenB]

@G-Rath seems I'm unable to actually assign you, so consider this message that 😀

[G-Rath]

@SimenB I did the upgrade to run lint against the codebase quickly to just get a sense of how much work would be involved, and right now the big one is the adding of Function to the defaults for ban-types.

I'm happy to go through them and have a go at replacing them with a stricter type, but if you could keep an eye out for usage of that type in PRs and ask authors to use a stricter type that would save me some work :)

(I don't expect there to be many, but did notice #10073, which I've put a comment on).

[SimenB]

Happy to take a PR adding Function to ban-types without the whole upgrade to v3. Might help landing this a bit iteratively if it's gonna be a big PR?

[G-Rath]

Totally! Landing iteratively is the way to go - I'll probably go package by package at this point, as there's quite a few uses of Function 😅

[SimenB]

@G-Rath now that TS 4 is out (and I'd love to use labeled tuples), any idea how close we are to being able to upgrade?

[G-Rath]

@SimenB getting there slowly :/

[SimenB]

I'd say this is done via #10437