C++: Add change note

Author: jketema

cpp/ql/lib/change-notes/2024-01-20-elifdef.md

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+---
+category: feature
+---
+* New classes `PreprocessorElifdef` and `PreprocessorElifndef` were introduced, which represents the C23/C++23 `#elifdef` and `#elifndef` preprocessor directives.
+* A new class `TypeLibraryImport` was introduced, which represents the `#import` preprocessor directive as used by the Microsoft Visual C++ for importing type libraries.