feat: support for type attribute on files

[wesleyorbin]

This small change adds support to natively run tests written as ES6 modules (without preprocessors) just by adding type: 'module' as an option on files. An example usage is below:

files: [
  { 
    src: [ 'my-test.js' ],
    type: 'module',
    served: true,
    included: true,
    watched: true
  }
]

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[wesleyorbin]

@googlebot I signed it!

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[gouthamrajmr]

Hi, is there a plan to merge this request? Our team needs it as well to support modules.