fix: externalize unified plugins by adding them to package.json

[YousefED]

I think this might have been breaking the sandpack examples. in vite.config.ts, we have the following setting:

// make sure to externalize deps that shouldn't be bundled
      // into your library
      external: (source: string) => {
        if (deps.includes(source)) {
          return true;
        }
        return source.startsWith("prosemirror-");
      },

We imported several packages that were not explicitly in "dependencies". this then caused the remark plugins to end up in our blocknote.umd.js built file. My guess is this broke the examples

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[YousefED]

confirmed to fix the dependency errors