Omit -jar when -cp or -classpath is used in custom java arguments

[NZSmartie]

In the case of using a 3rd-party mod loader. It's ideal to pass along a custom classpath to java.
However, if -jar is present, then the classpath is ignored.

This PR enables the use of the fabric mod loader through the custom java arguments input:

-Xincgc -server -cp ".\StarMade.jar;.\libs\fabric-loader.jar" -Dfabric.gameJarPath=.\StarMade.jar -Dfabric.loader.entrypoint=org.schema.game.common.Starter -Dfabric.side=client net.fabricmc.loader.launch.knot.KnotClient