Add support for prefixing initial-letter

[thomasmattheussen]

Fixes #1555.

Safari has required the -webkit- prefix for initial-letter across every version that implements it (9 through current 26.x), but Autoprefixer never adds it. caniuse-lite already has the correct data (the x prefix-required flag has been set for Safari since v9), it just wasn't wired up in data/prefixes.js.

Wired it up the same way as appearance/print-color-adjust:

// initial-letter
let prefixInitialLetter = require('caniuse-lite/data/features/css-initial-letter')

f(prefixInitialLetter, browsers =>
  prefix(['initial-letter'], {
    browsers,
    feature: 'css-initial-letter'
  })
)

Input:

.dropcap::first-letter {
  initial-letter: 2;
}

Output (targeting Safari):

.dropcap::first-letter {
  -webkit-initial-letter: 2;
          initial-letter: 2;
}

Added test/cases/initial-letter.css / .out.css fixtures and a dedicated test, plus wired the new case into the COMMONS list so it's covered by the existing suite-wide checks.