Move Flutter demo app into example/webcrypto_demo_flutter_app/

[Samarborkar]

Closes #283.

Moves the existing Flutter demo app out of example/ and into a named
subfolder, example/webcrypto_demo_flutter_app/, so that example/ can later
hold individual .dart snippet files for API documentation.

Folder name

Per the suggestion to consider the naming, I went with
webcrypto_demo_flutter_app — it clearly distinguishes the full Flutter demo
app from the small doc snippets that will live alongside it in example/.

Commits

1. refactor: the move itself — 130 files, tracked by git as 100% renames
   (no content changes), so history is preserved.
2. chore: fixes every reference to the old example/ path:
   • .github/workflows/test.yml — 6 working-directory / cd paths
   • example/webcrypto_demo_flutter_app/pubspec.yaml — path dependency
     ../ → ../../ (the app is one level deeper, so it must reach two levels
     up to the repo root)
   • tool/test.sh, tool/clean.sh — local helper script paths
   • analysis_options.yaml — narrowed the lint exclude to the app subfolder so
     future snippets in example/ remain analyzable
   • notes.md — updated a commented-out path reference

[jonasfj]

Sorry about the turnaround time, please do ping me when I forget.

I always have too many things cooking.

[Samarborkar]

Sorry about the turnaround time, please do ping me when I forget.

I always have too many things cooking.
@jonasfj
This was my first PR and I look forward contributing more, feeling good after my PR is merged. I'll keep checking but if you find or open any beginner level issues kindly tag me if you can , i'll be happy to contribute.

[jonasfj]

@Samarborkar we can probably find a lot more small things..

Especially, in the example space.

We should make sure all examples have static analysis.. ideally a form of testing.
We should make small examples for each and every method, lol 😂

Ideally, we should write tutorials and what not.

And there is still lots of testing to figure out. Exception testing, etc.