Register app as share target for audio files

[stekhn]

References:

• https://developer.chrome.com/docs/capabilities/web-apis/web-share-target
• https://web.dev/articles/workbox-share-targets
• https://github.com/chromeos/bridging-the-native-app-gap/blob/master/fugu-journal/src/js/service-worker.js

[stekhn]

Work in progress, but not yet complete.

Transcribe does appear as a target in the share dialog on Android, and it opens when an audio file is shared. However, the audio file is not being loaded into the app after it's opened. Debugging this issue is quite challenging since it can't be simulated with the PWA running on localhost, and installing and uninstalling the app for testing is tedious.