Fixes #8096

# Bug description

I was experimenting with adding extensions and almost went crazy trying
to make my demo extension work. It appeared that I was copying files
with Finder that creates hidden `.DS_Store` files which interfered with
Zed's loading logic. It assumes that `languages/` directory contains
only directories and never files and so it crashes when meets
`.DS_Store`. This makes any extension stop working after it has been
viewed via Finder

# Change

Check if path is directory when loading extension languages (so it will
skip .DS_Store files)