Incremental migration to concurrency checking (#1481)

* Initial draft

* Revise proposal and narrow it to @predatesConcurrency

* Add examples and describe @predatesConcurrency protocol behavior

* Rename `@predatesConcurrency` to `@preconcurrency`

* Clarify that using concurrency features enables Sendable checking.

* Simplify from 3 "concurrency checking modes" to 2.

We only need "minimal" and "strict"; the other differences are accounted
for by the language mode (warnings prior to Swift 6, errors thereafter)
and the behavior of the `@preconcurrency` attribute.

* Remove an extraneous space

* Add links to implementation, toolchains, pitch #3 thread

* Update proposals/NNNN-support-incremental-migration-to-concurrency-checking.md

Co-authored-by: Jon Shier <[email protected]>

* Assign number, kick off review

Co-authored-by: Ben Cohen <[email protected]>
Co-authored-by: Jon Shier <[email protected]>

Author: 3 people