docs(uri): add Compared to alternatives section

[ob-aion]

Summary

• Adds a ## Compared to alternatives section to the README, mirroring the pattern in @coroboros/clone.
• Compares @coroboros/uri against Node's native URL (WHATWG), uri-js, whatwg-url, and url-parse.
• Table rows are feature-presence only (RFC-3986 parse, §5.2 resolve, recompose, Punycode, RFC 6874 zone IDs, IP/domain validators, Sitemap validation, coded errors, deps) — no performance numbers, since the alternatives split between WHATWG and RFC-3986 models.
• Positioning paragraph names the gap (validation + Sitemap on top of RFC-3986 strict semantics) and the closest peer (uri-js).
• TOC entry added between Limitations and Contributing.

Test plan

• Render the README on github.com — anchor #compared-to-alternatives resolves from the TOC.
• Table renders with right-aligned status columns in GitHub-flavored markdown.
• No regressions in existing sections (Compliance, API, Errors, Limitations).