Serialize relative URLs explicitly

[samuel-williams-shopify]

Summary

• move explicit: from relative path calculation to URL serialization
• add to_s(explicit: true) and append(..., explicit: true) for relative URLs and references
• preserve absolute, root-relative, and parent-relative serialization
• remove explicit: from Path.relative and Relative#relative_to

There are two serialization contexts:

1. A field known to contain a URL, where the RFC-minimal relative reference is sufficient.
2. A mixed grammar where a string may be a URL or another kind of value, where the URL must be lexically self-identifying.

In the second context, explicit: true adds ./ to otherwise ambiguous same-directory references. Applying this at serialization preserves the distinction after URL composition and normalization.

This revises the API introduced in v0.17.0.

Testing

• BUNDLE_WITHOUT=maintenance bundle exec bake test (390 tests, 571 assertions)
• BUNDLE_WITHOUT=maintenance bundle exec rubocop