Support "groundcover" mapgen terrain

[Coolthulhu]

Currently every mapgen specifies own ground cover terrain. Many entries have differing amounts of grass vs dirt, some swamp mapgens have incorrect furniture placement (cattails on dry ground). This can be pretty jarring due to square nature of mapgen.

I see two options:

A fake terrain type is supplied. During mapgen, it is replaced with biome's default ground cover.
Advantages:

• Groundcover can be specified as fill_ter, mapping or placement, whichever is the most convenient
• Simple
• Probably faster than the alternative below

Each mapgen entry gains an optional (later possibly mandatory) field that describes what is it placed on. For example, most houses would be placed on a field, but swamp shack would be placed on swamp.
During mapgen, the types are processed sequentially: first a field is generated, then a house is generated on the field.
Advantages:

• Support for partially-jsoned mapgen (hardcoded swamp, jsonized shack on it)
• Support for furniture (cattails) due to the above
• Depending on implementation, could allow more "natural" results, like small house bordering a forest having more trees in its square

[BevapDin]

The second option sounds good, but is a bit more complicated: the second mapgen process must clear some of the first mapgen data and not just add its own data.

E.g. first mapgen adds shallow water tile and cattail on top of it. The second mapgen changes that into the floor of a house but it must also remove the cattail furniture. Current mapgen simply leaves the furniture as it (and it was initially set to f_null all over the map).

Same with items that had been spawned during the first mapgen and traps (don't want sinkholes inside the house).

[ZhilkinSerg]

Solved in #35627