Changes from v5.5.2:

vega

• Update movies-sort test scene to reflect new stable sorting.

vega-dataflow

• Add stableCompare method to Tuple utilities.

vega-encode

• Update SortItems transform to use stable comparator.
• Update Stack transform to use stable comparator.

vega-hierarchy

• Update HierarchyLayout to use stable comparator.
• Remove internal assignment of tupleid to tree nodes.

vega-parser

• Update sort parameter for SortItems transform.
• Update sort parameter for generated scale dataflows.
• Fix negative axis labelFlushOffset bug.

vega-transforms

• Update Collect transform to use stable comparator.
• Update Values transform to use stable comparator.
• Update Window transform to use stable comparator.