Add webpack condition, so webpack loads es6 module version

Even when handling the `exports` table, as done in `webpack` 5. `webpack` 5, at least at present, doesn't handle esm/cjs interop the same way as `node`, so we can;t use the same source as in `node`.

package.json

@@ -28,6 +28,7 @@
     "sideEffects": false,
     "exports": {
         ".": {
+            "webpack": "./tslib.es6.js",
             "import": "./modules/index.js",
             "default": "./tslib.js"
         },