Parse query string

Parse a URL query string into a dict. Strip a leading `?` if present. Split on `&` into pairs; each pair is `key=value` (split on the first `=` only) or a bare `key` with no `=` (value is the empty string). Percent-decode both keys and values (e.g. `%20` -> space). If the same key appears more than once, its value becomes a list of all occurrences in order; a single occurrence stays a plain string. Empty pairs (from `&&`) are skipped. An empty input yields an empty dict.