Add a module entry to the require exports condition

This should prevent exports-aware bundlers, such as Rollup with recent
versions of the node-resolve plugin, from including two copies of
Underscore.

Author: jgonggrijp

package.json

@@ -31,6 +31,7 @@
         "default": "./underscore-esm.js"
       },
       "require": {
+        "module": "./modules/index-all.js",
         "browser": {
           "production": "./underscore-umd-min.js",
           "default": "./underscore-umd.js"