feat: Add treemap component

Design system issue: gitlab-design#656 (closed)
Needed for gitlab#13008 (closed)

Basic hierarchy example	Basic flat example

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To do

• Split out options into defaults and configurable
• Add click handler
• Add legend
• Resolve console error Error: <tspan> attribute y: Expected length
• Calculate min / max based on data
• Add tests
• Add documentation
• See if possible to define rect size