url: speed up URLSearchParams

Parse query strings with indexOf instead of a per-character state
machine, skip ToString when values are already strings, cache
toString() until the list mutates, and join serialized pairs.

Signed-off-by: Yagiz Nizipli <yagiz@nizipli.com>
Assisted-by: Cursor
Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: anonrig