examples : update server.py to match github pages app [no ci] (#3004)

examples : update server.py to match github pages app [no ci] (#3004)

This commit updates examples/server.py which is used to serve the wasm
examples locally. The changes include:

- Added a redirect from the root URL to /whisper.cpp.
  So now accessing http://localhost:8000/ will redirect to
  http://localhost:8000/whisper.cpp/ which matches the url for the app
  deployed to github pages.

- Custom handling for coi-serviceworker.js to serve it to avoid
  and error in the console. This file is not strictly necessary
  for the local server to work as the headers are provided already but
  it is nice to not have an error in the console.

- Fixed the shutdown of the server to ensure it exits cleanly
  on Ctrl+C. Previously it would continue to hang onto the port even
  after the processed had exited.