[Fix] Prevent infinite recursion when a Window is exposed via js_api (#1838)

Exposing a js_api object that referenced a webview.Window (e.g. storing the
window on the API instance) made function discovery walk into window.native.
The native GUI object tree is effectively unbounded and many of its properties
return a fresh object on every access (AccessibilityObject.Owner.Accessibility
Object..., Bounds.Empty.Empty..., Style.Bold.Bold...), so the id() based cycle
guard never matched and the walk recursed until a RecursionError.

- Skip webview.Window.native during function discovery. Native objects cannot
  be bridged to Javascript and their tree is unbounded, so there is no reason
  to walk them.
- Add a JS_API_MAX_DEPTH setting (default 10) as a general backstop against any
  other pathological object graph, with a warning when the limit is reached.
- Document the new setting and add a regression test.