Merge buildData with element creation and the available child rect calculation moved to element

[stockiNail]

From @kurkle https://github.com/kurkle/chartjs-chart-treemap/pull/121#issuecomment-1258534332

buildData should be merged with element creation, and the available child rectangle calculation moved to element.

Something like

• determine main rectangle
• parse and create elements on this level
• recurse

So actually quite much the same as the buildData, expect creating elements on the fly.

Use data option instead of tree one.

[stockiNail]

Maybe the best solution is going to parse method of the controller, to split data and element creation.