[w3c/manifest] Feat(events): add BeforeInstallPromptEvent (#520)

Defines `BeforeInstallPromptEvent`, related Dictionaries, and `.prompt()` method as recently discussed in the relevant bug. 
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/520

-- Commit Summary --

  * Feat(events): add BeforeInstallPromptEvent

-- File Changes --

    M index.html (147)

-- Patch Links --

https://github.com/w3c/manifest/pull/520.patch
https://github.com/w3c/manifest/pull/520.diff

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