Included new weather 'Rain Storm' and appropriate sound category.

[L-3-on]

Summary

Content "Includes new type of weather 'Rain Storm' and sound category therefor"

Purpose of change

Adds a weather type similar to "Snowstorm" with a condition of windpower >= 15 but a temperature >= 33. Also integrating a separate sound category "rainstorm" it would be possible to improve sound packs with special audio file for that situations where there's a gale blowing and it's pouring.

Describe the solution

I searched the source files for all references of "Thunder", "WEATHER_THUNDER" and "outdoors_thunderstorm_env". After that, I took the following steps:

1. Added a class definition in "sounds.h" for the rainstorm channel
2. Expanded the audio channel check in "sound.cpp"
3. Included a new section for "Rain Storm" to all translation files
4. Added the weather type "rainstorm" to the "weather_type.json" file
5. Expanded the list of weather types to use in "regional_map_settings.json"
6. Improved documentation in "SOUNDPACKS.md" and "sound.h"

Describe alternatives you've considered

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Testing

Well, tbh. I have a bit of programing knowledge but never worked with C++ and MS Visual Studio so I simply couldn't figure out how to compile the source files. I therefore testetd the changes only the new weather type without a special audio channel definition: I included the "rainstorm" definition in "weather_type.json" and the new weather type to the "regional_map_settings.json". Those got loaded in the current experimental db8eb88 (tiles) without any problems and got used ingame without errors.

Additional context

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[L-3-on]

Btw. I included new sections to all translation files, but I didn't include translations for those languages!

[BrettDong]

Files in lang/po directory are managed by bots and must not be edited by human. You need to revert these changes.

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