feat: add named filter arguments and allow_false for default filter

[osteele]

Summary

• Add support for keyword arguments in filter expressions (e.g., {{ val | default: "fallback", allow_false: true }})
• Keyword arguments are parsed by the yacc grammar and collected into a map[string]any passed as a variadic trailing parameter to filter functions
• The default filter now accepts allow_false: true to preserve false values instead of replacing them with the default

Closes #42, closes #75

Changes

• expressions/builders.go: Added filterArgs and keywordArg types
• expressions/expressions.y: Extended filter_params grammar rule to accept KEYWORD expr pairs; changed union type from []valueFn to *filterArgs
• expressions/y.go: Regenerated from expressions.y
• expressions/context.go: Updated Context interface signature for ApplyFilter
• expressions/filters.go: Updated ApplyFilter to evaluate keyword args into a map[string]any and append as trailing argument
• expressions/filters_test.go: Updated tests for new filterArgs type; added keyword args test
• filters/standard_filters.go: Updated default filter to accept variadic kwargs and check for allow_false
• filters/standard_filters_test.go: Added test cases for allow_false behavior

Test plan

• All existing tests pass
• New tests for allow_false: true with false, nil, empty string values
• New test for keyword arg passthrough via ApplyFilter
• Existing filter tests updated for new filterArgs type