Implement Powershell Stack Workload

Original tracking issue: https://github.com/Azure/azure-functions-core-tools/issues/5322. Content copied over:

Add the PowerShell stack workload that wires the PowerShell worker into
`func init`, `func new`, and `func start` flows (language detection,
project scaffolding hooks, runtime resolution). Worker package itself is
tracked separately.

## Definition of Done

- [ ] Stack workload nupkg builds and publishes via the workloads pipeline.
- [ ] CI pipeline (public + official/signed) wired up for the workload.
- [ ] `func init --stack powershell` produces a valid project.
- [ ] Stack metadata surfaces in `func templates list` and `func --help`.
- [ ] Stack manifest validated by the schema in CI.

Start with the original tracking issue at Azure/azure-functions-core-tools#5322 and trace the existing workload integration points for func init, func new, and func start. Done means the workload package and public/official CI paths work, PowerShell initialization produces a valid project, metadata appears in templates and help, and the manifest passes schema validation.

Written by the indexing model from the issue text.