Common - Make FUNC(getVehicleIcon) check for strings without the .paa extension

[lambdatiger]

When merged this pull request will:

• Suggests a possible fix when the ".paa" extension is not included*.
  • This specifically would fix GM's vehicle icons not being retrieved.

* Apparently the ".paa" on a vehicle's icon config entry is optional.

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[lambdatiger]

The only issue with doing the fileExists check is that if an odd string is stored in icon such as "\a", it fileExits for "\a.paa" takes 20x longer to run (~100 µs for me), which might be too much overhead on a utility function.