Manifest - providing image

So in `mod_manifest.gd` we specify the image as
```
var image: CompressedTexture2D
```
Which in itself is a problem, as we are providing the input from `manifest.json` file, which should be a path to image file,
while CompressedTexture2D is *.ctex format. At least I didn't found the part of the code where we would try to load file from path.

Also I think it's better to leave var type as String as when exporting mod it will automaticly create import file and stor it as ctex file format. So only change in manifest `image` to `imagePath` and var type to String