refactor(lib): Add path separator helpers to PathUtil

[bobtista]

getExtension already had to find the last path separator and did it inline with maxPtr(strrchr(path, '/'), strrchr(path, '\\')). Several other call sites need the same thing, and some need the platform's own separator when building a path.

Now getNativePathSeparator(), isPathSeparator(), getLastPathSeparator() and getFileName() live in PathUtil.h, and getExtension uses them. getLastPathSeparator is maxPtr(strrchr(path, '/'), strrchr(path, '\\')), which is what the two getExtension overloads already did inline. No behavior change.

PathUtil.h is in Core/Libraries, which cannot depend on AsciiString in Core/GameEngine, so these are all const char* primitives. The AsciiString typed joining that #3107, #3108 and #3141 need is built on them at those call sites.

Todo:

• Replicate to Generals: N/A, it's in Core