Add missing "Contents" directory suffix for webots MacOS installations

[hoeftjch]

On macOS, Webots is installed as a .app bundle. The conventional WEBOTS_HOME points to the bundle root (e.g. /Applications/Webots.app), but all headers, sources, and libraries reside one level deeper under Contents/. The scripts and library.json assumed a flat layout, causing FileNotFoundError on macOS.

Changes

• scripts/create_webots_library.py: detect macOS and prepend /Contents to all paths derived from WEBOTS_HOME
• scripts/copy_webots_shared_libs.py: same fix for the dylib paths in SHARED_FILES_LIST
• library.json: fix -L flag from lib/Webots/lib/controller to lib/Webots/lib (the actual directory create_webots_library.py copies the dylib into)

Note: The webots-controller executable will also expect WEBOTS_HOME to simply point to the *.app file and would fails with the following error when adding "Contents/" to the WEBOTS_HOME variable and would fail with this error:

dyld[5780]: Library not loaded: @rpath/Contents/lib/controller/libController.dylib
  Referenced from: <2029B01F-9D80-3E00-90C0-C23650F528D8> /Users/jhoeft/projects/RadonUlzer/.pio/build/LineFollowerSim/program
  Reason: tried: '/Applications/Webots.app/Contents/Contents/lib/controller/libController.dylib' (no such file), '(null)/libController.dylib' (no such file)

[BlueAndi]

The documentation of Webots mention this here, so I think the fix is ok:
https://cyberbotics.com/doc/guide/compiling-controllers-in-a-terminal?version=master