Allow font face values added via addBase

[ben-rogerson]

This PR fixes an issue with @font-face being added in a tailwind plugin using addBase.

Example

// tailwind.config.js

const plugin = require("tailwindcss/plugin");

const basePlugin = plugin(({ addBase }) => {
  addBase({
    "@font-face": {
      fontFamily: "myFont",
      fontWeight: 400,
      fontStyle: "normal",
      src: `url('./fonts/myFont.ttf')`,
    },
  });
});

module.exports = {
  plugins: [basePlugin],
};

Output before

Using GlobalStyles to render the base styles:

// ...
'@font-face' { } 

Output after

// ...
@font-face {
  src: url('./fonts/myFont.ttf');
  font-style: normal;
  font-weight: 400;
  font-family: myFont;
}