This PR updates TailwindCSS and ShadCN to the newest versions, and also changes the overall setup for the @blocknote/shadcn package.

Currently, we import Tailwind styles in @blocknote/shadcn/src/style.css. This generates CSS from all of the utility classes that Tailwind detects in the code, and that has multiple issues:

• Creates duplicate CSS rules in applications which already import Tailwind styles themselves.
• BlockNote doesn't respect the Tailwind/ShadCN theme of the parent application.
• We have to scope the BlockNote Tailwind CSS to prevent conflicts (which is why we have things like the bn- prefix).

In practice, this doesn't make sense. We expect @blocknote/shadcn to be used in an app that already uses Tailwind, and probably ShadCN. Therefore, it doesn't make sense to include Tailwind at all in the package, and should instead rely on the parent application to handle importing the Tailwind styles. This way, all of the above issues are resolved.

There is one caveat to this - Tailwind only generates CSS based on the utility classes it finds in your code, which is why we previously needed to include that generated CSS in the built files. Since the @blocknote/shadcn package is buried inside node_modules, Tailwind doesn't check the code there, and there is no way to guarantee that the parent app would use all of the utility classes that BlockNote uses to generate all the necessary CSS. However, Tailwind v4 introduced the @source directive for this exact purpose, so the parent app can now point Tailwind to the code in @blocknote/shadcn and generate all the additional CSS needed.

So this PR removes the Tailwind dependency from the @blocknote/shadcn package, and the parent app is instead expected to handle it. Even if the parent app doesn't use ShadCN and only uses Tailwind, the worst case is that you have to paste some CSS from here to set up the theme. In our case, the ShadCN example is responsible for importing Tailwind styles and setting up the ShadCN theme.