[himura467]

Closes #15

• Adds Nack::QueryParser with parse_query, parse_nested_query, build_query and build_nested_query.
• parse_query and build_query follow WHATWG application/x-www-form-urlencoded; parse_query returns [name, value] pairs.
• parse_nested_query and build_nested_query handle a[b], a[] and deeper names, keep malformed names flat, and raise Nack::ParamError on conflicting shapes.
• Raises Nack::ParamError beyond param_limit (4096) or depth_limit (32).
• Instances are frozen and shareable across Ractors.