fix: wrap custom parser errors with flag name context

[privatenumber]

Summary

• Custom type parser errors are now wrapped in a TypeError with the flag name for context
• Original error preserved via cause for programmatic access
• Example: Flag "--port": Invalid number: abc instead of just Invalid number: abc

Test plan

• Existing parser error tests updated to expect new format
• New test verifying original error preserved via .cause
• 90 tests passing

BREAKING CHANGE: Custom type parser errors are now wrapped in a TypeError. Error messages change from <message> to Flag "--<name>": <message>. Original error available via .cause.

[github-actions]

📊 Package size report   0.9%↑

File	Before (Size / Gzip)	After (Size / Gzip)
dist/index.cjs	3.3 kB / 1.7 kB	4%↑3.4 kB / 3%↑1.7 kB
dist/index.mjs	3.3 kB / 1.7 kB	4%↑3.4 kB / 3%↑1.7 kB
Total (Includes all files)	28.0 kB / 11.3 kB	0.9%↑28.3 kB / 1%↑11.4 kB
Tarball size	9.5 kB	3%↑9.8 kB

Unchanged files

File	Size (Size / Gzip)
dist/index.d.ts	6.2 kB / 2.2 kB
LICENSE	1.1 kB / 667 B
package.json	1.4 kB / 624 B
README.md	12.8 kB / 4.5 kB

_{🤖 This report was automatically generated by pkg-size-action}

[privatenumber]

🎉 This PR is included in version 5.0.0-beta.2 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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