Snapshots: normalizers, obsolete snapshot report, strictness options, more types #120

Part of #101. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced, suspected = from reading the code.

• Normalizer / redactor hooks to blank out timestamps, UUIDs and tokens. Today this only exists for JSON-LD /.well-known/genid/ ids.
• A report of obsolete snapshots, i.e. files no test used during a run.
• Configurable strictness: an option for strict JSON comparison (key order and formatting; today assertJsonStringEqualsJsonFile canonicalizes), and tidy options.
• More snapshot types: YAML, CSV, image/binary.