[rcore] Add filtering folders to LoadDirectoryFilesEx()/ScanDirectoryFiles() (#4302)

* Add filtering folders in ScanDirectoryFiles and ScanDirectoryFilesRecursively

Add define FILTER_FOLDER for that purpose
Fix folder names matching filter being added to result

* Move FILTER_FOLDER define to internals of rcore and document option in comment

src/raylib.h

@@ -1126,7 +1126,7 @@ RLAPI bool ChangeDirectory(const char *dir);                      // Change work
 RLAPI bool IsPathFile(const char *path);                          // Check if a given path is a file or a directory
 RLAPI bool IsFileNameValid(const char *fileName);                 // Check if fileName is valid for the platform/OS
 RLAPI FilePathList LoadDirectoryFiles(const char *dirPath);       // Load directory filepaths
-RLAPI FilePathList LoadDirectoryFilesEx(const char *basePath, const char *filter, bool scanSubdirs); // Load directory filepaths with extension filtering and recursive directory scan
+RLAPI FilePathList LoadDirectoryFilesEx(const char *basePath, const char *filter, bool scanSubdirs); // Load directory filepaths with extension filtering and recursive directory scan. Use "/DIR" in the filter string to include directories in the result
 RLAPI void UnloadDirectoryFiles(FilePathList files);              // Unload filepaths
 RLAPI bool IsFileDropped(void);                                   // Check if a file has been dropped into window
 RLAPI FilePathList LoadDroppedFiles(void);                        // Load dropped filepaths