feat: init new plugin system (#3323)

plugins/hello-world/src/app.tsx

@@ -0,0 +1,17 @@
+import { IconButton, Tooltip } from '@affine/component';
+import { AffineLogoSBlue2_1Icon } from '@blocksuite/icons';
+import { useCallback } from 'react';
+
+export const HeaderItem = () => {
+  return (
+    <Tooltip content="Plugin Enabled">
+      <IconButton
+        onClick={useCallback(() => {
+          console.log('clicked hello world!');
+        }, [])}
+      >
+        <AffineLogoSBlue2_1Icon />
+      </IconButton>
+    </Tooltip>
+  );
+};