Enable Span-based URL implementation

[jrflat]

Enables the faster Span-based URL implementation from #1844 by removing the FOUNDATION_SWIFT_URL_V2 build guard. See more implementation details in #1844

Motivation:

Many common URL operations such as path manipulation or resolution (currently) involve repeated string allocations, percent-decoding, and re-parsing. By working directly with Span(s) of the underlying bytes and replacing just the encoded path during path manipulation (ensuring that nearby ranges and path-related flags are updated correctly), we see 5-8x speedup in use cases like appending or deleting path components.

The new backing type also consolidates and shares code from the Swift NSURL/CFURL implementation, which improves parsing, validation, encoding, and resolution performance (2-3x speedup), and makes it easier to maintain consistent behavior across all the types.

Modifications:

Remove FOUNDATION_SWIFT_URL_V2 guard so that the _URL backing class is used.

Result:

Ubuntu 24.04 URL Performance vs. Baseline

URL operations are significantly faster, and URL, NSURL, and CFURL share the same Swift code for parsing, percent-encoding, file path initialization, and resolution.

Bug fixes/compatibility behaviors:

• Prevents double-encoding of already-percent-encoded characters when another invalid character exists in the component (deviating from URLComponents encoding to match original CFURL byte encoding behavior and the behavior of most other RFC 3986 and WHATWG parsers). Resolves Double encoding of query component #1190.
• Allows [ and ] unencoded in the path, query, and fragment via Allow square brackets in Swift CFURL path, query, and fragment #1802, matching WHATWG behavior.
• standardized now preserves leading .. segments in relative paths so they're not removed until resolution.
• File path colon-encoding in the first relative path segment is more lenient when the character preceding the colon cannot be part of a scheme (e.g. %43: is not mistaken for a scheme).
• appendingPathExtension rejects extensions containing invalid characters that would be problematic when decoded (e.g. spaces or Unicode directional override characters) instead of storing them percent-encoded.

Testing:

• Benchmarks for file and non-file URL path manipulation.
• Unit tests on Darwin, Linux, and Windows.

[jrflat]

@swift-ci please test

[jrflat]

@swift-ci please test

[jrflat]

Squashed and rebased to pick up bug fixes for cross-repo testing

[jrflat]

@swift-ci please test

[jrflat]

@swift-ci please test