Upcoming and experimental features are supported via command-line flags
and also in the SwiftPM manifest. Introduce it as an experimental
feature so that it can be enabled via SwiftPM without having to resort
to unsafe flags.
The `StrictConcurrency` experimental feature can also provide a
strictness level in the same manner as `-strict-concurrency`, e.g.,
`StrictConcurrency=targeted`. If the level is not provided, it'll be
`complete`.
Note that we do not introduce this as an "upcoming" feature, because
upcoming features should be in their final "Swift 6" form before
becoming available. We are still tuning the checking for concurrency.
