ParseConstant: discover itemoptionconstant_* files instead of a hardcoded suffix list

[Zintixx]

Summary

ParseConstant iterated a hardcoded constantSuffix array to build filenames. This replaces it with a prefix scan over the archive's file list — the same idiom already used by ItemParser, AchieveParser, MapParser, AdditionalEffectParser, FunctionCubeParser, etc.

foreach (PackFileEntry entry in xmlReader.Files.Where(entry =>
        entry.Name.StartsWith("itemoption/constant/itemoptionconstant_"))) {
    string xml = Sanitizer.RemoveEmpty(xmlReader.GetString(entry));
    ...

Why

• Flexible: auto-discovers every itemoptionconstant_* file (including custom merge files) with no suffix list to maintain.
• Robust: skips absent files instead of throwing via GetEntry(...).First(...) when a listed suffix has no file.
• Consistent: matches the dominant enumeration pattern across the other parsers.

Removes the now-dead constantSuffix array.

No behavioral regression: the ingest consumer keys constant options by (code, grade) and throws on true duplicates regardless of order, so the enumeration-order change (archive FileList order vs the fixed array) doesn't affect correctness.

Bumps PackageVersion to 2.4.20 (2.4.19 is already published).

🤖 Generated with Claude Code

[Zintixx]

Retargeting to MS2Community/Maple2.File (correct upstream).