Align parsing of custom resolver options, rename arg (#42392)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42392

Follow-up to https://github.com/facebook/react-native/pull/42333 following internal feedback. We are now aligning this to match the [`metro build` command](https://github.com/facebook/metro/blob/702e1b8fc7ab8b973bcd53f1a41f7e797cbf7dca/packages/metro/src/commands/build.js#L85-L91). This also improves validation on parsing (done after initial `commander` arg parsing as variadic string).

Changelog: [Internal]
(same as https://github.com/facebook/react-native/pull/42333)

Reviewed By: motiz88

Differential Revision: D52911017

fbshipit-source-id: 54049aa20c9db344a0f485fddf62fb267e672376