[LUA API] change filesys functions naming

[JimB40]

As File System #630 will grow over time including next methods I suggest to move existing functions to own 'class/object' and maintain LUA API naming scheme. And then we put them in separate group called File System Functions So: dir -> fs.getDir fstat -> fs.getFIleStat

Once done we should move & rename (alias for legacy) chdir() -> fs.changeDir()

@raphaelcoeffic @riwanou @jfrickmann