[fix][compiler] fix for loop declaration
[enh][compiler][interpreter] add build in functions to check if var has value
[enh][interpreter] add assert function
[fix] add check-if-var-is-defined into rand() and fix bug
[enh][doc] improve german documentation
[enh][preprocessor] implement if conditions in the preprocessor
[fix][preprocessor] add forgotten rule and update syntaxhighlighter