Check for .tres material if no .material file exists

[the-sink]

Just a quick fix to add the ability for the plugin to utilize .tres materials if it doesn't find a .material. Not sure if it's a reasonably good implementation or if you wanted to do this differently, but I was having issues with #15 since I use .tres exclusively (for version control purposes), and wanted to get that working to use for my project!

[codecat]

This looks good, thank you! 🎉

The only thing I worry about is the .tres file not actually being a material, what happens then? But considering we are looking in the textures folder, it might not cause much issues, so we can think about that later. 👍