Start v6
v6 filter syntax

Fixes #178
Fixes #191
Fixes #189
Fixes #168
Fixes #160
Fixes #159
Add geometry_type enum back
v6 line gap/offset properties
Punting on line-offset (one direction) for now
Ditch zoom dashes, closes #175
Add a default for text-font
Filters cannot reference constants (fixes #196)
Filter operators are strict (fixes #194)
Regen docs
Group by layer rather than render/style (fixes #193)
render ⇢ layout, style ⇢ paint (fixes #166)
Reorder a few things

Order here is going to determine order in pretty-printed
output.
add min/max
raster min/max's
add period key, for rotation properties
fix spacing
spacing
spacing
add schema keys