Add missing main field for @tailwindcss/browser

[RobinMalfait]

This PR adds a main and browser field for the @tailwindcss/browser package.

In the package, we do have the exports field setup, which is an alternative to the main field according to the docs:

The "exports" provides a modern alternative to "main" …

— https://docs.npmjs.com/cli/v10/configuring-npm/package-json?v=true#exports

However, if you look at the unpkg link: https://unpkg.com/@tailwindcss/browser, it tries to load the index.js file. This is probably a bug in the unpkg resolver.

That said, if you look at other CDNs such as esm.sh, it does resolve correctly: https://esm.sh/@tailwindcss/browser

According to the npm docs:

If main is not set, it defaults to index.js in the package's root folder.

— https://docs.npmjs.com/cli/v10/configuring-npm/package-json?v=true#main

This explains why unpkg is trying to load the index.js file.

Additionally, the npm docs also mention the browser field:

If your module is meant to be used client-side the browser field should be used instead of the main field. This is helpful to hint users that it might rely on primitives that aren't available in Node.js modules. (e.g. window)

— https://docs.npmjs.com/cli/v10/configuring-npm/package-json?v=true#browser

So this PR also adds that field just to be sure.