added support for IF-THEN-ELSE-END expression

build.gradle

src/main/java/de/tlc4b/analysis/Typechecker.java

src/main/java/de/tlc4b/prettyprint/TLAPrinter.java

src/main/java/de/tlc4b/tla/Generator.java

src/main/java/de/tlc4b/tla/TLAModule.java

src/test/java/de/tlc4b/prettyprint/SyntaxExtensionsTest.java