Alpha-less colors

[ayrton]

When certain core plugins have been disabled (eg. backgroundOpacity) jit mode currently does not output alpha-less colors like default mode does.

With the following config:

module.exports = {
  corePlugins: {
    backgroundOpacity: false,
  },
}

Before:

.bg-gray-100{
	--tw-bg-opacity: 1;
	background-color: rgba(243, 244, 246, var(--tw-bg-opacity));
}

After:

.bg-gray-100{
	background-color: #f3f4f6;
}

Fixes #3963

[ayrton]

This was backported from tailwindlabs/tailwindcss-jit#196

[adamwathan]

Thanks!