Convert recursive functions to iterative traversal

[swhitty]

SVGs with deep nested nodes (100+ deep) fail to parse and render when performed off main thread in Swift Concurrency.

This PR converts the recursive functions within the SwiftDraw to use iterative traversal within;

• Parser when building DOM
• LayerTree.Builder when building LayerTree
• CommandGenerator when creating array of rendering commands

Fixes #72

[codecov]

Codecov Report

Attention: Patch coverage is 71.52778% with 41 lines in your changes missing coverage. Please review.

Project coverage is 83.66%. Comparing base (63a6201) to head (a69e747).

Files with missing lines	Patch %	Lines
SwiftDraw/LayerTree.Layer.swift	4.54%	21 Missing ⚠️
SwiftDraw/LayerTree.Builder.swift	73.80%	11 Missing ⚠️
SwiftDraw/LayerTree.CommandGenerator.swift	83.01%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   83.89%   83.66%   -0.23%     
==========================================
  Files          82       82              
  Lines        7344     7384      +40     
==========================================
+ Hits         6161     6178      +17     
- Misses       1183     1206      +23     

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