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

[foxblock]

Right now LoadDirectoryFiles and LoadDirectoryFilesEx only return either all folders and all files (when no filter is passed) or a list of files filtered by file extension.
For use in a file browser (like raygui\examples\custom_file_dialog) it is useful to gather a list of files with a particular extension and also all folders in a location (so the user can navigate further).

This change implements this by adding a FILTER_FOLDER define.
The define can be used as a filter and will match all folders. The contents of the define are chosen as to never match a file extension on Linux and Windows.

So for example you can do
LoadDirectoryFilesEx(%RAYLIB_SRC_DIR%, FILTER_FOLDER";.c", false);
which results in

..\raylib\src\external
..\raylib\src\platforms
..\raylib\src\raudio.c
..\raylib\src\rcore.c
..\raylib\src\rglfw.c
..\raylib\src\rmodels.c
..\raylib\src\rshapes.c
..\raylib\src\rtext.c
..\raylib\src\rtextures.c
..\raylib\src\utils.c

As a side effect this fixes one "bug" (at least I assume it was not intended behaviour):
LoadDirectoryFiles(Ex) would previously return folders which also matched the file extension. So LoadDirectoryFilesEx(RAYLIB_SRC_DIR, ".c", false); would match a folder with a name like "code.c".
With this change it does not do that any more (only if FILTER_FOLDER would have been passed as well).

Existing calls to LoadDirectoryFilesEx should not be affected otherwise and return the same results as before.

Btw. I did not see a place to add tests, but I am happy to provide some if needed.

[raysan5]

@foxblock Thank you very much for the improvement, I will think about the best tag name for directories... but /DIR looks good to me for now.