New rule: ban index signatures entirely

[danielnixon]

Index signatures lead to all sorts of unsoundness and you're better off using a real Map in almost every case.

[danielnixon]

This rule should recommend https://immutable-js.github.io/immutable-js/docs/#/Map or TypeScript's ReadonlyMap type if the builtin Map really must be used.

[danielnixon]

This just landed a few hours ago: typescript-eslint/typescript-eslint@d7dc108

Maybe the best approach here is to teach that rule a new never option (akin to https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/consistent-type-assertions.md#assertionstyle)

[danielnixon]

This is less of an issue now that https://devblogs.microsoft.com/typescript/announcing-typescript-4-1-beta/#no-unchecked-indexed-access is a thing but probably still worth doing.

[danielnixon]

Even with noUncheckedIndexedAccess, there are other reasons to prefer real Map types: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#objects_vs._maps