feat(output): add a unified output result

[hogo6002]

Refactored the HTMLResult into a unified structure that can be reused by other output formats. This new structure presents results hierarchically, grouping vulnerabilities by ecosystem, source, and package. Compare to the original vulnerabilityResult, it contains more information, such as vulnerability count (called/uncalled/fixable/unfixable/severity), package/vulnerability fix information, etc.

Follow-up PRs:

• Merge this into the v2 branch (via chore: Update v2 to main #1396)
• Leverage this structure to generate a table package-view output.
• Update html.go and HTML templates to use the OutputResult.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.31646% with 78 lines in your changes missing coverage. Please review.

Project coverage is 69.05%. Comparing base (c80f395) to head (b3bcc7c).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/output/output_result.go	75.16%	61 Missing and 14 partials ⚠️
internal/utility/severity/severity.go	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1397      +/-   ##
==========================================
+ Coverage   68.96%   69.05%   +0.08%     
==========================================
  Files         185      186       +1     
  Lines       17934    18237     +303     
==========================================
+ Hits        12368    12593     +225     
- Misses       4903     4965      +62     
- Partials      663      679      +16     

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[another-rex]

Thanks! Added some comments.

[hogo6002]

I'm going to merge this PR first to create the package view for the table output. Feel free to leave comments, and I'll address them in a separate PR.