feat: add fmt.Stringer implementation for Node

[ianlewis]

Description:

Add a fmt.Stringer implementation for Node. The String method prints the full tree structure of the Node and its children in a human friendly format similar to the GNU tree program.

Related Issues:

Fixes #98

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Update documentation if applicable.
• Add a note in the CHANGELOG.md if applicable.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.09%. Comparing base (e14f106) to head (32c6480).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
+ Coverage   83.85%   86.09%   +2.24%     
==========================================
  Files           5        5              
  Lines         353      374      +21     
==========================================
+ Hits          296      322      +26     
+ Misses         40       34       -6     
- Partials       17       18       +1     

Files with missing lines	Coverage Δ
parser.go	91.93% <100.00%> (+1.64%)	⬆️

... and 2 files with indirect coverage changes

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[Copilot]

Pull Request Overview

This PR adds a fmt.Stringer implementation to the Node type, enabling it to render tree structures in a human-readable format similar to the GNU tree program. The implementation recursively formats nodes and their children with proper indentation and tree-drawing characters.

Key changes:

• Added String() method to Node type implementing fmt.Stringer interface
• Implemented recursive fmtNode() helper function for tree formatting
• Added comprehensive test coverage for the new functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
parser.go	Implements String() and fmtNode() methods for tree-formatted output of Node structures
parser_test.go	Adds test case validating the tree formatting output with nested child nodes

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