BooleanLiterals in EL do not have a getValue method

BooleanLiteral do not have a getValue method since EMF names methods returning a boolean according to Java's naming convention isValue().

java.lang.UnsupportedOperationException
	at de.evoal.languages.model.el@1.0.0-20230211.015315-107/de.evoal.languages.model.el.impl.LiteralImpl.getValue(LiteralImpl.java:48)