fix(url): make URLSearchParams record constructor spec compliant

[mattermoran]

Fixes the issue where byte replacement characters could result in duplicate entries when constructing URLSearchParams from a record.

While current implementation follows the spec to the word:

Otherwise, if init is a record, then for each name → value of init, append (name, value) to query’s list.

using params_append() directly happens before the USVString conversion so that keys that are supposed to collide after conversion can be added as separate entries.

By using params_set() we get the correct behavior without having to normalize the record beforehand.

Extending maybe_consume_sequence_or_record with separate sequence and record handlers allows applying different behavior.

Passes 2 remaining tests for URLSearchParams constructor.

[andreiltd]

Thanks!