fix: 🐛 take care of case when fs is empty object (#152)

In browser environment with current package.json `fs` is empty object,
which results in `constants` being undefined.

constants.js

@@ -1,12 +1,14 @@
-try {
-  module.exports = require('fs').constants
-} catch {
-  module.exports = { // just for envs without fs
+const constants = { // just for envs without fs
   S_IFMT: 61440,
   S_IFDIR: 16384,
   S_IFCHR: 8192,
   S_IFBLK: 24576,
   S_IFIFO: 4096,
   S_IFLNK: 40960
-  }
+}
+
+try {
+  module.exports = require('fs').constants || constants
+} catch {
+  module.exports = constants
 }