feat(tilesets): Support widget calculations on tilesets

[donmccurdy]

Changes:

• Migrate local calculations from C4R
• Port local calculations to TypeScript
  • In some cases the types were not obvious, particularly required vs. optional parameters and overloaded types (see: SortColumns). I've used strict TS types where possible, and generally tried to make parameters required for internal functions so that defaults are applied only once at top-level calls.
• Migrate unit tests from C4R
• Add WidgetTilesetSource class
• Refactor WidgetSource class hierarchy:
  • WidgetSource
    • WidgetRemoteSource
      • WidgetTableSource
      • WidgetQuerySource
    • WidgetTilesetSource
• Add tileset example

Remaining:

• test with H3 and quadbin tilesets
• publish alpha release
• include Web Workers