Adopt @microbit/eslint-config (#25)

Replaces the legacy .eslintrc.cjs and per-project plugin set with the
shared flat config, moving to ESLint 9. eslint-plugin-storybook stays a
project dependency as its version tracks Storybook's release cycle; the
one-line flat/recommended addition isn't worth a shared config export
until more Storybook conventions accumulate.

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

- Type the postMessage listeners' event.data, previously any, via
  narrow local types that the existing per-message casts build on.
- Rework MakeCodeBlocksRendering's empty-code case to derive the
  rendered output instead of setting state synchronously in the effect
  (react-hooks set-state-in-effect).
- SnippetAnswerTypes uses (string & {}) to keep the named literals in
  autocomplete rather than a union the string member absorbs.
- Type info()'s response resp field, previously any via the intersection
  with EditorMessageResponse.
- Mark intentionally unawaited promises with void, drop async from
  functions with no await, and remove now-unnecessary assertions.

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']];