Use the Script mod type for mods including native and Squirrel scripts

[Xanfre]

This PR should move script detection fully into its own mod category rather than having scripts detected as generic resources, encompassing all native scripts (OSMs) in the root mod directory and all Squirrel scripts in the sq_scripts subdirectory.

It's also worth noting that loose Squirrel script files in the root directory should preferably not be detected. The Squirrel module exclusively looks in sq_scripts for scripts upon being loaded, so any that are located elsewhere will always be ignored. While the native script directory paths are configurable with the script_module_path variable, all mods currently (and likely ought to continue to) assume that only the root directory is a valid location.

[pshjt]

I'm not too familiar with mod setups outside of SS2, so I'm not 100% sure if this is true for all of the Dark engine games, so I also asked the SS2 experts here: https://www.systemshock.org/index.php?topic=4790.msg157104#msg157104

The manager could indeed try to detect the script_module_path as it already reads the install.cfg or dark.cfg if it exists, but my current assumption is that it's not to be touched, and mods rather use the existing mod folders automatically rather than requiring the user to edit a default folder. The exception is the movie_path as it's commonly updated while being absent from cam_mod.ini.

So, I'll just wait for some feedback like I mentioned above.

Will release a patch version soon with everything included.