feat: Add experimental setup future flag (#14361)
## Summary
Add the root-only `futureFlags.experimentalSetup` opt-in as a foundation
for managed repository setup.
```diff
"futureFlags": {
+ "experimentalSetup": true
}
```
- Default the flag to `false` and preserve strict boolean and
unknown-key validation.
- Keep the Rust parser, manual TypeScript forms, generated schemas, and
public types aligned.
- Document the experimental status and unstable prelaunch `turbo.lock`
format.
- Preserve existing run and task-hash behavior with the flag omitted,
disabled, or enabled, including when `turbo.lock` is present.
This is a parser/type foundation only: it adds no setup command, `setup`
configuration key, or lock processing. Follow-up PRs must gate each
setup, execution, activation, and hashing surface behind this flag.
Closes TURBO-6295.
Part of TURBO-6275.
