docs: Update resolverMainFields for vanilla React Native

[jd20]

Issue: #255

What I did

Add "browser" to the mainResolverFields, so as not to break polyfills that depend on it.

An alternative approach would be to override the default config's mainResolverFields, but this requires a much bigger change, that I assume some teams may not be comfortable with. Since the default for pretty much all React Native apps should be the same, I think just adding "browser" is a much less invasive change. If a team has customized their mainResolverFields already, then it should be pretty clear what they need to do, when updating their metro.config.js. Just for reference, below is the altnerative solution:

const {getDefaultConfig} = require('metro-config');

module.exports = (async () => {
  const {
    resolver: {resolverMainFields},
  } = await getDefaultConfig();
  return {
    transformer: {
      getTransformOptions: async () => ({
        transform: {
          experimentalImportSupport: false,
          inlineRequires: false,
        },
      }),
    },
    resolver: {
      resolverMainFields: ['sbmodern', ...resolverMainFields],
    },
  };
})();

How to test

Run through the v6 instructions, everything should still work same as before.

• Does this need a new example in examples/native? no, I did update the existing example though.
• Does this need an update to the documentation? included

[dannyhw]

After checking the metro config packages these are the default values anyway and I think its the easier way with fewer changes for the user so I'm ok with this. Maybe we can provide the alternative in the docs somewhere if it becomes necessary.

Thanks for your contribution 🙇

[dannyhw]

gist with the script in has now been updated also