Local font source is not recognized as valid

[Rarst]

Only font faces having src: url() are treated as valid, so src: local() ones would always be considered invalid and dropped, even if they are detected as critical fonts.

@font-face {
  font-family: "Alegreya-fallback";
  size-adjust: 101%;
  src: local("Times New Roman");
}

https://github.com/GoogleChromeLabs/critters/blob/a590c05f9197b656d2aeaae9369df2483c26b072/packages/critters/src/index.js#L568-L587

Related #112