feat: Add PathEq matcher for filesystem paths

This commit introduces a new Google Test matcher, PathEq, for comparing
std::filesystem::path objects. The matcher normalizes paths by
converting them to strings and replacing backslashes with forward
slashes, ensuring consistent comparisons across different platforms
(Windows and POSIX).

This matcher is useful for cross-platform testing where file paths may
have different representations.

Includes a set of unit tests to validate the matcher.

Change-Id: Ic839320334e16c20b8ba6b8cae23b023f21824b3