Web IDE: Create `.gitkeep` in new empty subdirectories to allow committing empty directory

Proposal

A customer requests the ability to commit empty directories from the Web IDE. This is currently not possible in either the legacy Web IDE nor the new VS Code-based editor, presumably due to upstream git's empty directory limitation.

However, one can commit "empty" directories in the UI because we create a .gitkeep in the directory. Creating a .gitkeep when commiting from the Web IDE will allow users to commit empty directories as well as bring the Web IDE behavior in line with the UI behavior.