Permit boolean variables and expressions in SCEst statements

[a-sr]

The extension of the Esterel grammar into SCEst does not widen the admissible scope of trigger expressions and still requires signals. As a consequence, an abort cannot be triggered by a boolean variable, which should be possible in SCEst.

The SignalOrTickReferenceExpression grammar rule should be overridden to include variable references. However, this will transiently permit present checks on variables, which should be excluded by separating the rules or implementing validator checks.