Optionally preserve empty path segments.

[samuel-williams-shopify]

Summary

• Add preserve_empty: to Protocol::URL::Path#simplify and #simplify!.
• Preserve repeated separators while still resolving dot segments when explicitly requested.
• Keep separator collapsing as the default behavior.
• Document explicit composition with path.normalize.simplify(preserve_empty: true).

This intentionally does not add an option to Relative#normalize!, keeping its canonicalization contract narrow while allowing callers to compose the lower-level operations when original empty segments are significant.

Verification

• 374 tests pass with 546 assertions.
• RuboCop reports no offenses.

[ioquatix]