Ruleset with mixed exports

[curiosus42]

User story. As a developer, I can use js modules rulesets with mixed exports (named and default), so that I can improve my development experience.

Is your feature request related to a problem? Description: It is not possible to use a .js file with mixed exports as a ruleset for spectral. Error:

Entry module "ruleset.js" is using named and default exports together. Consumers of your bundle will have to use `chunk["default"]` to access the default export, which may not be what you want. Use `output.exports: "named"` to disable this warning
Error running Spectral!
Use --verbose flag to print the error stack.
Error #1: exports is not defined

Cause: The rulesets are bundled with rollup.js to commonJS. On named and default (mixed) exports rollup.js binds default exports to exports.default. When only a default export is used rollup.js binds the default export to module.exports.

Describe the solution you'd like It is possible to use js modules with mixed exports as a ruleset.

Suggestion: Pass a export object instead of an module object on bundle load as a fallback.

Additional context Example: working bundle output, with only default export:

// bundledSource_default.js

'use strict'

var spectralFunctions = require('@stoplight/spectral-functions')

var ruleset = {
  rules: {
    'example-rule': {
      description: 'example rule',
      severity: 0,
      given: '$.paths',
      then: {
        field: 'abc',
        function: spectralFunctions.truthy,
      },
    },
  },
}

module.exports = ruleset

Not working bundle output, with mixed exports:

// bundledSource_mixed.js

'use strict'

Object.defineProperty(exports, '__esModule', { value: true })

var spectralFunctions = require('@stoplight/spectral-functions')

const abc = 12

var ruleset = {
  rules: {
    'example-rule': {
      description: 'example rule',
      severity: 0,
      given: '$.paths',
      then: {
        field: 'abc',
        function: spectralFunctions.truthy,
      },
    },
  },
}

exports.abc = abc
exports['default'] = ruleset