…meter properties on private constructors (#21515)

## Summary

A private parameter property on a private constructor doesn't need an explicit type annotation:

- The constructor signature is collapsed to `private constructor();` — the parameter is never emitted.
- The parameter-property class member is emitted as `private readonly name;` — type-erased.

Since the parameter's type does not appear in the `.d.ts` output in either place, no type is needed. This matches `tsc --isolatedDeclarations`. Non-private parameter properties on a private constructor still require a type, because the class member keeps the type.

## Example

```ts
export class GreyhoundServer {
  private constructor(
    private readonly logger = new ConsoleLogger(['greyhound-server']),
  ) {}
}
```

Before: `TS9011: Parameter must have an explicit type annotation`.
After: no error — emits `private readonly logger;` and `private constructor();`, matching tsc.

## Test plan

- [x] Added snapshot fixtures covering both the no-error case (private param property) and the still-errors case (public param property on private constructor)
- [x] `cargo test -p oxc_isolated_declarations`

Closes #21503 (case 1)