Added landcover=grass as grass

[pietervdvn]

For the moment, landcover=grass does not render as grass. As this is subtly different then landuse=grass, this is a valuable addition.

For example, here we would like to map the grass within a residential area. By using 'landcover', we can correctly use this within a residential area.

Rendering won't change visibly here:

[kocio-pl]

I would like to implement it for some time already, but there is a big debate with different views on the topic, so please look at #2548 first and relate to a discussion.

It also desn't look like your code is complete - you need to edit project.mml too to select the objects from database, and this is probably not the proper rendering test, just a screenshot of landuse=grass areas. Please try to install rendering environment, probably this way:

https://github.com/gravitystorm/openstreetmap-carto/blob/master/DOCKER.md

[kocio-pl]

Since the discussion and coming to some conclusions is the basic problem, I will close this PR now. I hope we will find some common ground at last, so adding the code will make sense.