AST tightening: `PrivateIdentifier` can only be LHS of `BinaryExpression[operator='in']`

### Suggestion

We are planning to enforce the following invariants in our AST for `BinaryExpression`:

- `left` can only be a `PrivateIdentifier` if `operator: "in"`, so the following is a parsing error:
  ```ts
  #a + 1;
  ```

@sosukesuzuki @fisker Do these look safe to you?