Updates to align with Vega 5.4.

Additions

top level legend positioning with leX and leY.
interior and exterior filtering of topoJson meshes (topojsonMeshInterior and topojsonMeshExterior)
dnMinSteps and dnMaxSteps added to trDensity options
new trKde transform for 1-d KDE from a distribution.
new trRegression transformation function.
new trLoess locally estimated regression function
new vGradient and vGradientScale functions for setting gradient fills/strokes.
previous and next value window value operations (woPrevValue and woNextValue)
arrow file format indicator (arrow) for loading binary apache arrow files.