Proposal: Support mirroring features

[skyclouds2001]

What information was incorrect, unhelpful, or incomplete?

It'll be helpful to support features which has vendor-prefixed features and helps maintaining those css features.

What did you expect to see?

support mirror data

Do you have any supporting links, references, or citations?

see #864 (review)

Do you have anything more you want to share?

possible ways can be:

speicified directly as a string

{
  "-webkit-user-select": "user-select"
}

speicified via mirror key on mirroring data

{
  "-webkit-user-select": {
    "mirror": "user-select"
  }
}

speicified via mirrors key on mirrored data

{
  "user-select": {
    "mirrors": ["-webkit-user-select"]
  }
}