Enhance test coverage of FEEL built-in functions

Majority of classes in module kie-dmn-feel in package org.kie.dmn.feel.runtime.functions can be tested by unit tests. Having such tests we can cover nearly 100% of these classes.

While coding these tests I found and fixed these bugs and problems:

• Rounding problem in EvalHelper.getBigDecimalOrNull() method - The method used .doubleValue() call which produced rounding errors.
• AnyFunction and AllFunction didn't check if the items in the list parameter are all Booleans (they must be according to the spec.)
• ConcatenateFunction didn't check for nulls in the list parameter
• EqualsFunction didn't compare BigDecimals with compareTo() method (which checks just values and ignores scale)
• MinFunction and MaxFunction didn't check if the items in the list parameter are comparable
• NumberFunction - split of one error message into two
• ReplaceFunction - NPE when having replacement param as null
• StringFunction and CodeFunction contained duplicated code
• SublistFunction - it was possible to input negative length as parameter
• ReplaceFunction now handles regexp flags

I will create a PR with new tests and fixes for mentioned things.