Add an immutable URL type for parsing, resolving and building URLs

Add an immutable URL type to Objo's standard library so developers can parse addresses, inspect their components, resolve relative links, and build query strings without assembling URLs by hand.

URL values should have consistent equality and hashing, making them suitable for dictionary keys and hash sets while preserving meaningful differences in paths, queries, and fragments. Changes should return new URL values, leaving the original unchanged.

The type should support absolute URLs across schemes, provide non-throwing parsing and resolution methods, and integrate with URLConnection while retaining its existing String overloads.