[WIP] Fire nerf

The basic idea here is to add a small, non-hazardous fire to model the very first stages of lighting a fire.
These fields might even be relatively long-lived if the fuel being consumed isn't particularly volatile.

Things like molotovs and flamethrowers would still create full-size fire fields immediately, mut matches and lighters would now make these little flames, which would need to be nurtured into a full-sized fire.

I started on this a while back and got distracted from it, posting in case someone wants to follow up.

- [ ] Add growth stages for small fires, including promotion to full-size fires.
- [ ] Add effects for small fires? (just a teensy bit of hotfoot?)
- [ ] Switch firestarters over to making these fields instead of full-sized fires.
- [ ] Make fire spread using these instead of spawning full-sized fire fields.

Possibly related, possibly belongs in its own PR:
- [ ] Add a "hot air" field produced by fires and other heat sources, these would meander around much like smoke, but would be harmless except for elevating the temperature of the area.  These would model simple building heating.
Looks like a line's missing here:
```diff
+            "fd_flame",
```