Add matrix syntax support in shell

[Copilot]

• Add matrix syntax support for shell interface using MATLAB-like notation
• Implement matrix parsing with square bracket notation
• Support rational numbers, variables, and unary operators in matrices
• Add comprehensive tests for matrix syntax
• Update GUIDE.md with BNF grammar and examples
• Fix pipeline issues: resolve clippy warnings and formatting errors
  • Fixed needless_return warning in is_valid method
  • Added #[allow(clippy::to_string_trait_impl)] attributes for existing ToString implementations
  • Applied cargo fmt for code formatting

Original prompt

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This section details on the original issue you should resolve

<issue_title>Matrix syntax in shell</issue_title>
<issue_description>Add support for matrices creation in shell.
TODO:

• Write BNF rules for a matrix
• Add those rules to parser
• Add unit tests
• Add documentation in GUIDE.md

Example of syntax (based on MATLAB):

% Matrix with two rows. Each row has two elements. First row [1, 4], second [3, 2]
[1 4; 3 2]
% Matrix with two rows. Each row has three elements. First row [-1/2, 5/4, 5/2], second [1/2, 13/17, -9/2]
[-1/2 5/4 5/2; 1/2 13/17 -9/2]
% Matrix with one row. This row has three elements. It contains of [a, -3/2, x]
[a -3/2 x]

This syntax does not allow arithmetic expressions apart from scalars and variables, as e.g., expression

[3/4 -2/1]

can be interpreted either as [3/4, -2/1] or [-5/4].</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #43

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[tudny]

@copilot Fix the pipeline

[Copilot]

@copilot Fix the pipeline

Fixed the pipeline issues. The problems were:

1. Clippy warnings about direct ToString implementations - added allow attributes to maintain existing code structure
2. A needless return statement in the is_valid method - removed the explicit return
3. Code formatting issues - ran cargo fmt

All tests are now passing and clippy/formatting checks are clean. Commit: c0a8b5f