Environment Type System

**Summary**: Add environment type awareness to AZD with support for customizable environment types and type-specific behaviors.

**Acceptance Criteria**:

- [ ] Add `--type` flag to `azd env new ` command
- [ ] Support predefined environment types (dev, test, staging, prod)
- [ ] Allow custom environment types per organization
- [ ] Generate type-specific `azure.yaml` files (e.g., `azure.dev.yaml`, `azure.prod.yaml`)
- [ ] Store environment type metadata in environment configuration
- [ ] CLI commands to list and manage environment types
- [ ] Validation of environment type constraints