Move internal modules to “internal” folder.

.internal/stackDelete.js

@@ -0,0 +1,18 @@
+/**
+ * Removes `key` and its value from the stack.
+ *
+ * @private
+ * @name delete
+ * @memberOf Stack
+ * @param {string} key The key of the value to remove.
+ * @returns {boolean} Returns `true` if the entry was removed, else `false`.
+ */
+function stackDelete(key) {
+  const data = this.__data__;
+  const result = data['delete'](key);
+
+  this.size = data.size;
+  return result;
+}
+
+export default stackDelete;