adds a sandy margin to ocean beaches

[I-am-Erk]

Summary

Features "Adds sand to ocean beaches"

Purpose of change

Oceans were added in #70070 but are currently just a sudden batch of salt water where once there was grass.

Describe the solution

Adds sand to beaches and adjusts the spawning of ocean shores such that:

1. ocean shores are less 'noisy' than lakes, with a smoother boundary into water
2. the transition from shore to water is more gradual, going from surrounding terrain to sand to water over most of an OMT
3. amount of sand can be specified in json settings

Describe alternatives you've considered

I'm considering some alternative beach formations depending on the aliased terrain but I think that's probably better handled with map extras. I would like to manage the additional contents of beaches through map specials and map extras that allow us to dynamically turn chunks of the shoreline into rocks and other features, and that will come after this basic step.

Testing

I used ascii here because t_swater_surf isn't ready yet. The cyan line is the "surf" zone, which will be a transitional art tile showing waves crashing on the sand (I hope) or at least a nice boundary if I can't get that to work properly. The yellow is the sandy beach, the rest is salt water.

There are some artifacts to work out, such as the boundary with rivers. I also don't like that it's a constant unchanging bar of sand. However this at least opens things up to adding some map extras and stuff that will improve the look.

Additional context

#70173 will add some extra terrain and perhaps some map extras to spawn here.

[ADekema]

These could be added to portions of rivers and lakes as well.

[I-am-Erk]

These could be added to portions of rivers and lakes as well.

The mapgen as it works currently is pretty rigid and won't do that very well, but it may be possible eventually. They'll likely want their own systems

[Procyonae]

These could be added to portions of rivers and lakes as well.

The mapgen as it works currently is pretty rigid and won't do that very well, but it may be possible eventually. They'll likely want their own systems

You could feasibly do it with a mutable map but I'm not sure if that's really very sensible

[github-actions]

Auto-requesting reviews from non-collaborators: @Night-Pryanik