Strict mode. Show a warnings if a class cannot be generated with current config

[user753]

For example if I have this config

module.exports = {
    mode: 'jit',
    content: ["./src/**/*.{html,js}"],
    theme: {
        colors: {
            'blue': '#1fb6ff',
            'purple': '#7e5bef'
        },
        extend: {},
    },
    plugins: [],
}

and I try to use text-red

  <h1 class="text-red">
    Hello world!
  </h1>

tailwindcss should show a warning class "text-red" cannot be generated.