fix: also check for "EISDIR" in mkdirp

Fixes webpack#10544.

Though the `EISDIR` error code is not mentioned in the POSIX standard,
FreeBSD and memfs both throws this error when trying to run `mkdir` on
`/`. So essentially we should treat it the same as `EEXIST`.

References:
* https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=59739
* streamich/memfs#326

lib/util/fs.js

@@ -141,7 +141,7 @@ const mkdirp = (fs, p, callback) => {
 					});
 				});
 				return;
-			} else if (err.code === "EEXIST") {
+			} else if (err.code === "EEXIST" || err.code === "EISDIR") {
 				callback();
 				return;
 			}