Add getProp(itemName, propName) to replace getItemProps(itemName).propName
Update getBlockPropsAll to use getProp()
Add experimental dynamic properties (function values)

If a property value is set to a function, then it will be called in
getProp(), with 'this' as the props object, and the block offset index
as the argument if applicable (e.g., with registerBlocks()).
Remove 'name' property to avoid mutating props, move to blockIndex2Name