Silently fail to 'enter' regular files

[duganchen]

Silently fail to "Enter" regular files. Entering only makes sense for directories.

This fixes https://github.com/sayanarijit/xplr/issues/653#issue-1806818324

I don't know Rust at all, so I make no claims to the code quality. But I have tested this change and it does work.

[sayanarijit]

I think we can use focused_node's built-in is_dir and symlink.is_dir, without needing a system call.

[duganchen]

I changed it to use focused_node().is_dir() and tested it. With both regular files and directories, and with symlinks of regular files and directories.

[sayanarijit]

Thanks.