bugfix(map): Write and scan the map cache with the platform separator

[bobtista]

• Closes Map cache is not written on non-Windows builds #3101
• Depends on refactor(lib): Add path separator helpers to PathUtil #3105 and bugfix(map): Guard the fallback map display name against a missing separator #3106, whose commits are included here until they merge
• Depends on bugfix(filesystem): Preserve native separators in archive listings #3141, which must merge first or the archived maps are listed under a mixed path

writeCacheINI does filepath.concat('\\') before appending the file name, and loadMapsFromMapCacheINI and loadMapsFromDisk build their paths the same way. Off Windows that character lands inside the file name:

before:  <user maps dir>/Maps\MapCache.ini
after:   <user maps dir>/MapCache.ini

Now all three go through getPathInDirectory and getLastPathSeparator, so the cache is written where it is read from and the map path shape is checked against whichever separator the listing actually used.

INIMapCache reads that same cache, and built the fallback display name for maps without a localized name tag with name.reverseFind('\\') + 1. With no backslash in the name that is nullptr + 1. Now it reads the leaf through getLastPathSeparator, the same guard applied to the MapUtil fallback in #3106.

Todo:

• A client with no existing cache writes maps/MapCache.ini at the correct path
• A custom map with no localized name tag shows its display name
• Replicate to Generals — N/A, it's in Core