Expose Node resolver's main fields on config

[intrnl]

Is your feature request related to a problem? Please describe.
Svelte expects using the svelte field in package.json for component libraries

Describe the solution you'd like
Expose mainFields in config, so that plugins are able to use it too. Perhaps it would be best if the entire resolver config is exposed?

Describe alternatives you've considered
User importing component libraries by path instead of relying on the fields on package.json

<script>
  import { Router } from 'svelte-router/lib/index.js';
</script>

Additional context

• Are there any notes about how config.createResolver is supposed to work? I know it's internal, but I tried this and it seemed to match just about anything.

  resolver = config.createResolver({
    extensions: [],
    mainFields: ['svelte'],
  });