Forbid TypeScript's private modifier in favour of hash names

[Mrtenz]

private fields and methods in TypeScript are not truly private. They can still be accessed in runtime. Using hash names (e.g., #name) makes them truly private, enforced by JavaScript itself.

[legobeat]

$ node ./scripts/validate-configs.js

Error: Computed snapshot differs from the existing snapshot for the following package(s). Take a new snapshot and try again.

    @metamask/eslint-config-typescript

error Command failed with exit code 1.

[Gudahtt]

LGTM!