POM: add main class
POM: add main class

The org.scijava.sjep.Main class will be the entry point when executing
the JAR using "java -jar scijava-expression-parser-1.0.0.jar".

In theory, it will also be available from the Maven build via
"mvn -Pexec", but that feature is not currently working as intended.

scijava/scijava-expression-parser