Add support for the tailwindcss/plugin export

[thecrypticace]

This PR adds support for the tailwindcss/plugin import which has historically been used to define custom plugins:

import plugin from "tailwindcss/plugin";

export default plugin(function ({ addBase }) {
  addBase({
    // ...
  });
});

This also adds support for plugin.withOptions which was used to define plugins that took optional initilization options when they were registered in your tailwind.config.js file:

import plugin from "tailwindcss/plugin";

export default plugin.withOptions((options = {}) => {
  return function ({ addBase }) {
    addBase({
      // ...
    });
  };
});

We've stubbed out support for the config argument but we're not actually doing anything with it at the time of this PR. The scope of this PR is just to allow people to create plugins that currently work using the raw function syntax but using the plugin and plugin.withOptions APIs. Support for config will land separately.

[thecrypticace]

There doesn't appear to be a way for tsup to compile a default export to module.exports in my testing so we came up with this + two separate entry points in the tsup config. I like this because it feels like it's more clear that it's a workaround. We might be able to do something like this for the postcss plugin too but would need to test.

[RobinMalfait]

I wonder if this is better or not 🤔

Suggested change

	if ('__isOptionsFunction' in plugin) {
	if (Object.hasOwn(plugin, '__isOptionsFunction')) {

[RobinMalfait]

The in one is cleaner if TypeScript is happy with it.