Fix highlighting of escaped $< in CMake regex strings (#5028)

[Quan Zhuo (quanzhuo)]

This change addresses item #5028

This changes visible behavior

The following changes are proposed:

• Fix CMake syntax highlighting so escaped $< sequences in quoted regex strings are not treated as generator expressions.
• Add a visual syntax test case for MATCHES "\\$<".
• Add a changelog entry for the syntax highlighting fix.

Other Notes/Information

Normal generator expressions such as $<CONFIG:Debug> are still highlighted as generator expressions.