v4 migration

[ansis]

• $type values Update $type filter values to match GeoJSON/VectorTile spec mapbox/mapbox-gl-style-spec#69

[ansis]

Started on this in https://github.com/mapbox/mapbox-gl-style-lint/tree/v3.1

• figure out how to better handle versions, 3.1 isn't a separate file in mapbox-gl-style-spec
• convert halo width units fix text-halo-width units mapbox/mapbox-gl-style-spec#66

[mourner]

figure out how to better handle versions, 3.1 isn't a separate file in mapbox-gl-style-spec

As we decided to drop minor versions (per discussions with @yhahn), we should consider changes like this as v3 patches (that become obsolete once v3 is released). We can migrate existing v3 styles once instead of keeping a migrate script, or we can keep them as separate scripts and then drop when no longer relevant.

[ansis]

• promote glyphs to a top level element Promote glyphs to a top level element mapbox/mapbox-gl-style-spec#77

[mourner]

So we've decided it'll be v4, right?

[mourner]

Lets add tasks to the master list in pull: #16