Feature modelling language add support for xor, or and ?

[SimonOsterheider]

Description

Adapted the FeatureModelLexer to parse "xor", "or" and "?" tokens (The implementation is a bit clunky as we parsed the letters individually, as searching for the whole token did not work at first, but seems to work for other groups).
Modified FeatureModel.bnf to allow a ? token after every featurename and allow an or/xor to be used with at least to featurenames after it.
Modified FeatureModelHighlighter.flex to parse xor, or and ? tokens.
Added different colors in FeatureModelSyntaxHighlighter.java for the operator

Related Issue

#42

Motivation and Context

Extend the feature modelling language to allow a feature to be marked as optional and allow multiple features to be grouped by an or, or xor to follow Clafer definition.

How Has This Been Tested?

• Tests are created and passing
• Manual testing
  We tested it by adding multiple nested or and xor statements and optionality tokens in the snake project and see if any errors occured

Demo:

feature_modelling.mp4

Types of changes

• Pipeline changes
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have updated the CONTRIBUTORS file.
• I have updated the CHANGELOG.