Add container machine for managing persistent Linux VMs (#1662)

## Type of Change
- [ ] Bug fix
- [x] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
`container` runs each workload in an ephemeral VM, so there's no
built-in way to keep a persistent Linux environment you can log into and
work in. `container machine` adds one.

A container machine is a lightweight, persistent, and integrated Linux
environments that feel like an extension of your Mac, created from
standard OCI images with a familiar UX. The login user matches your host
account with passwordless `sudo`, your home directory is mounted inside
the VM, and each machine keeps its filesystem and runs the image's own
init system (such as`systemd` or `openrc`).

```bash
container machine create alpine:3.22 --name my-machine
container machine run -n my-machine # interactive shell
container machine set -n my-machine cpus=4 memory=8G
```

Subcommands: `create`, `run`, `list` (`ls`), `inspect`, `set`,
`set-default`, `logs`, `stop`, `delete` (`rm`); `m` aliases `machine`.
Docs added to `docs/command-reference.md` (Machine Management) and
`docs/how-to.md` ("Use container machines").

## Testing
- [x] Tested locally
- [x] Added/updated tests
- [x] Added/updated docs

Signed-off-by: Raj Aryan Singh <rajaryan_singh@apple.com>
Co-authored-by: Jaewon Hur <jaewon_hur@apple.com>
Co-authored-by: John Logan <john_logan@apple.com>
Co-authored-by: Michael Crosby <michael_crosby@apple.com>
Co-authored-by: Eric Ernst <eric_ernst@apple.com>
Co-authored-by: Danny Canter <danny_canter@apple.com>

Sources/ContainerCommands/Image/ImagePrune.swift

@@ -24,7 +24,7 @@ extension Application {
         public init() {}
         public static let configuration = CommandConfiguration(
             commandName: "prune",
-            abstract: "Remove all dangling images. If -a is specified, also remove all images not referenced by any container.")
+            abstract: "Remove unused or all images")
 
         @OptionGroup
         public var logOptions: Flags.Logging