Allow mods to modify option_slider

### Is your feature request related to a problem? Please describe.

Mods would greatly benefit from being able to add own `option_slider` entries or to modify existing ones. Examples would include:

- the rural mapgen mod so it could turn down the city size quite a bit by default
- Innawood because no cities at all (or maybe tiny tribal villages?)
- Aftershock Exoplanet to reduce NPC spawning chance

and many more.

### Solution you would like.

After modifying mods in the mod manager, any new `option_slider` JSON objects need to be evaluated. Because it is used very early on, there needs to be a special, but optional, file for that, just like modinfo.json. For simplicitys sake I would suggest options.json or something similar.

### Describe alternatives you have considered.

_No response_

### Additional context

_No response_

Read the mod manager flow and the existing option_slider handling, using modinfo.json as the stated precedent and options.json as the proposed configuration file. Determine how mods can add or modify sliders before they are evaluated, and verify that the resulting settings affect the listed mapgen and NPC-spawning examples.

Written by the indexing model from the issue text.