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