Migrate some default values to config

[kanitw]

• Migrate default value to config (If all default values are in config and if we specify them explicitly, there is no need to maintain schema.instantiate.)
  • text -- add config.mark.text for placeholder text
  • sort -- just stick in the code that undefined means ascending
  • data -- Hardcode default data format + auto parse file extension from the url
• Migrate config.cell.grid* to config.facet.grid.*
• Remove config.scene because it's not that useful.
• Fix Revise config.cell #1057 - Add config.unit and config.facet.unit & remove config.cell
  • For faceted chart, config.facet.unit will override config.unit. This allows us to add stroke to each cell/unit of the faceted chart by default.
• add _config to the model and change signature for compileMarkConfig, compileStack
• Use model.encoding() rather than model.spec().encoding
• Rename Bin to BinProperties because in fact Bin = boolean | BinProperties + remove all default: undefined

The only default value left in the schema is scale, which will be eliminated in a follow-up PR