Introduce syntax highlighting for Boolean values

[frademacher]

Currently, the Boolean values true and false receive no syntax highlighting in the modeling languages' editors.

They are defined as possible values of the BOOLEAN terminal

terminal BOOLEAN returns ecore::EBoolean: 'true' | 'false';

in the grammar definition of the Domain Data Modeling Language.

Note that the highlighting needs to be implemented/tested for all modeling languages that make use of the PrimitiveValue modeling concept and the corresponding grammar rule.