[w3c/manifest] feat(window): add window.onbeforeinstallprompt (#517)

Part 1, will send bits that can go in easily... then link everything together. 

Can folks please review these as they come in.  Will try to keep them small and self contained. 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/517

-- Commit Summary --

  * feat(window): add window.onbeforeinstallprompt

-- File Changes --

    M index.html (13)

-- Patch Links --

https://github.com/w3c/manifest/pull/517.patch
https://github.com/w3c/manifest/pull/517.diff

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/pull/517