remove unused %filter directives and update token matching documentation

[ehwan]

Description

This Pull Request simplifies the grammar specification and the code generator by completely removing two unused and legacy syntax directives: %trace and %filter.

Key Changes

1. Removed %filter Directive:
   • Removed %filter parsing rules, arguments, macro substitution, and verification logic.
   • Deleted the unused diagnostic errors MultipleFilterDefinition and FilterNotDefined from rusty_lr_parser and rusty_lr_buildscript.
   • Removed the filter function implementation and directive from the calculator example (example/calculator).
2. Documentation Updates (SYNTAX.md, README.md):
   • Removed all references to %filter, and $filter.
   • Enhanced the %token definition section in SYNTAX.md with detailed instructions and examples on using native Rust structural pattern matching (destructuring) for wrapped/struct tokens.

[gemini-code-assist]

Code Review

This pull request removes the %filter directive and all its associated logic, errors, tests, and documentation from the parser. Instead of using a custom filter function, token classification now relies directly on native Rust pattern matching, which simplifies the codebase and documentation. I have no further feedback to provide as there are no review comments.

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