Depend only on FoundationEssentials when possible

[t089]

The binary size on non-darwin platforms suffers quite a bit, when you import full Foundation. The ecosystem is moving to adopt only FoundationEssentials when possible.

I've already prepared two PRs for the modelgen and model repos to replace Foundation with FoundationEssentials when possible.

For the main repo client the change is also quite straight-forward: diff. It requires switching from FoundationProcess to swift-subprocess, replacing NSLock with NIOLock and a couple of other Foundation to stdlib or swift-algorithm replacements.

WDYT?

[iabudiab]

@t089 I've merged one and left a comment on the other PR. Will take care of client afterwards. The change there and here in client makes perfect sense.

I was also thinking about raising the baseline to Swift 6 (i.e. drop Swift 5.10) or at lease use a separate manifest or branch for Swift 5.10. But that is a tale for a separate issue.