Adopt @microbit/eslint-config (#10)

Replaces the legacy .eslintrc.cjs and per-project plugin set with the
shared flat config, moving to ESLint 9. As in makecode-embed,
eslint-plugin-storybook stays a project dependency since its version
tracks Storybook's release cycle.

Fixes for the type-aware rules the shared config enables:

- Type the frame driver listener's event.data, previously any, via a
  narrow local type the existing per-message casts build on.
- Mark the intentionally unawaited handleWorkspaceSync call with void
  and drop async from initialProjects callbacks with no await.

The lockfile is regenerated as npm could not resolve the ESLint major
bump in place.

Author: microbit-matt-hillsdon

.eslintrc.cjs

@@ -0,0 +1,4 @@
+import microbit from '@microbit/eslint-config/react';
+import storybook from 'eslint-plugin-storybook';
+
+export default [...microbit, ...storybook.configs['flat/recommended']];