Flag constant-vs-constant comparisons as `SemanticError` in `SurveyModel.validateExpressions`

[JaneSjs]

T27766 - FormPath — a browser-local checker for SurveyJS conditional logic
https://surveyjs.answerdesk.io/internal/ticket/details/T27766

In condition expressions (visibleIf, enableIf, requiredIf, etc.), report a SemanticError when both sides of = == / != are literals (e.g. 'property-use' == 'residential'). Such rules never depend on answers and are almost always authoring mistakes from free-text input.