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