Add backdrop-filter to Autoprefixer webkit

[pedrobslisboa]

Description

This PR adds the missing backdrop-filter for webkit cases.

How to test

• Run make test and ensure that it passes:

let backdrop_filter =
  test "backdrop_filter" (fun () ->
      prefix_one_declaration
        (CSS.backdropFilter [| `blur (`px 30) |])
        "-webkit-backdrop-filter: blur(30px); backdrop-filter: blur(30px);")