feat: add support for fill modifiers in binary operations

[andriibeee]

fix VictoriaMetrics/VictoriaMetrics#10598

Summary by cubic

Added support for fill modifiers (fill, fill_left, fill_right) in binary operations to set default values when one side is missing. This expands query syntax and improves control over join behavior.

• New Features
  • Parser recognizes fill, fill_left, and fill_right modifiers with numeric values, including negatives, inf, and NaN.
  • fill(x) sets both FillLeft and FillRight; fill_left(x) and fill_right(x) set sides independently.
  • Works alongside existing on()/group_* modifiers and is treated as a reserved identifier.
  • Output formatting includes the fill modifiers via appendModifiers.
  • Added tests for case-insensitive syntax, value parsing, and normalization.

^{Written for commit 14411c8. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

[codecov]

Codecov Report

❌ Patch coverage is 76.11940% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.39%. Comparing base (0dc0e2d) to head (14411c8).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
parser.go	73.33%	8 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   89.69%   89.39%   -0.31%     
==========================================
  Files          11       11              
  Lines        2969     3035      +66     
==========================================
+ Hits         2663     2713      +50     
- Misses        209      217       +8     
- Partials       97      105       +8     

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