time for major ESM change

[Jimmy Wärting]

• find-up & load-json-file are now both ESM (just need to update dep), now would be a grate time to upgrade this pkg-conf.

• Also a good opportunity to fix your todo's
  https://github.com/sindresorhus/pkg-conf/blob/147a41b22f10fd23ea25381e8056d551468445e7/index.js#L59-L62

• Could also take the opportunity to change Object.assign to spread { ...x }, typescript understands it better...