Add support to find dead symlinks

[cgzones]

Find dead symbolic links by reusing the --type empty flag.

[sorairolake]

I think this can be achieved by running fd -t symlink -L. So I don't think there is a need for another way to do this.

See also #1298.

GNU findutils describes about find -L -type l which is similar to fd -t symlink -L:¹

symbolic link; if '-L' is in effect, this is true only for broken symbolic links. If you want to search for symbolic links when '-L' is in effect, use '-xtype' instead of '-type'.

So it may be useful to explain how to search broken links in README.md or fd(1).

Footnotes

1. https://www.gnu.org/software/findutils/manual/html_node/find_html/Type.html ↩