Concurrency-Safe Notifications (#822)

* Propose concurrency-safe notifications

* Minor correction to proposal template

* Minor clarifications

* Fix return statement of example EventDidOccur.makeMessage()

* Remove static requirement from Message.isolation and provide a table to better explain Message/Notification conversion

* Fix typo in ObservationToken definition

* Fix typo in using WillLaunchApplication.self

* Update proposal to remove generic isolation, enable static member lookup, and provide a default value for Message.name.

* Remove object parameter in makeNotification() example

* Update SF-0010 status to active review

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Co-authored-by: Charles Hu <[email protected]>