Beta release of v3.0.0-beta.15

Breaking changes:

• Use hyphenation (step-after) not camel-case (stepAfter) for interpolation curve names.
• Move scale "scheme" to an object-valued property of "range".
• Drop "index" scale, no longer needed with new scheme support.
• Update JSON configuration format: move config.mark.marktype blocks to config.marktype.
• Replace View.state with View.getState, View.setState methods.
• Migrate to use new "$schema" property.

Other changes:

• Add custom version of "sequential" scale to support piecewise domains.
• Add scale "interpolate" property for color space interpolation.
• Add scale "domainMid" property for simplifying diverging color ramps.
• Add ordinal, ramp, diverging, and heatmap default color scale ranges.
• Update legend entry and gradient methods to support interpolation.
• Suppress initial zero-sized legend symbols.
• Add "strokeDash" scale support to legends.
• Add treePath and treeAncestors hierarchy expression functions.
• Update hierarchy transforms to support configurable node key field.
• Export new interpolate and interpolateRange methods from vega-scale.
• Add basic runtime state snapshot support to vega-runtime.
• Add initonly parameter to Formula transform to derive values for new tuples only.
• Fix Pulse visit reflow bug.
• Optimize parameter modification tracking for better performance.
• Update PreFacet transform to propagate mod tuples.
• Add interpolate curve options to area.vg.json example spec.
• Add tree-radial-bundle example spec; update other examples.
• Add legends-interpolate example spec.
• Update example specs to use $schema property.
• Update example specs to use new range scheme.