Recognize the ".spdx3.json" extension when converting from SPDX 2 to SPDX 3 #278

Currently, we have:

To convert from SPDX 2 to SPDX 3.0.1:

• use the file extension .jsonld.json or .jsonld;

• or add the options for the from and to file types:

  java -jar tools-java-2.0.5-jar-with-dependencies.jar Convert hello.spdx hello.spdx.json TAG JSONLD

We should allow .spdx3.json also.