Jsonize swim speed mutation modifier as per walk cost mutation modifier

[ghost]

Summary

SUMMARY: Features "Jsonize swim speed mutation modifier as per walk cost mutation modifier"

Purpose of change

Jsonize swim speed mutation modifier so new mutations can make use of it, instead of checking for specific mutations in swim_speed() in player.cpp
there is already movecost_modifier member for mutations that apply a modifier to walk speed calculations, so its consitent to have swim speed behave the same instead of flat reductions per specific mutations.

Describe the solution

jsonize the movecost_swim_modifier and add to applicable mutations, load it ingame, and then modify swim_speed() to apply that multiplier.

Describe alternatives you've considered

N/A

Testing

Tested and got similar results to what currently happens with swim speed with the flat modifier checks for fins and scales etc.

Additional context

N/A

[kevingranade]

This pull request has been mentioned on Cataclysm: Dark Days Ahead. There might be relevant details there:

https://discourse.cataclysmdda.org/t/swim-speed-moddifiers/22382/8