`hasOwnProperty` is unsafe if called on an object which may potentially
have its own or an inherited property called "hasOwnProperty" or which
has a null prototype. Add a polyfill for the modern alternative and
remove the ESLint suppression.

 - Replace various uses of `hasOwnProperty` with `hasOwn` polyfill. In
   tests we can use `Object.hasOwn` directly, since tests only run in
   modern browsers.

 - Remove obsolete `Object.assign` polyfill in `parse-json-config.js`.
   IE 10/11 have been unsupported for long enough that I think we can
   stop worrying about it in the boot script.