feat: initial implementation of processorAsync

[johnnyreilly]

This PR is a basic implementation of the Async custom processors proposal: #204

As discussed on the linked issue the API is:

const results = await replaceInFile({
  files: 'path/to/files/*.html',
  processorAsync: async (input, file) => { 
    const asyncResult = await doAsyncOperation(input, file);
    return input.replace(/foo/g, asyncResult) 
  },
})

What do you think? Is this what you had in mind?

[adamreisnz]

Thanks, looks good! Just keen to get the one error case tested as well so we retain good coverage of the code.

[adamreisnz]

Thank you!

[adamreisnz]

Released in 8.3.0