Add globalObject: 'this' to webpack config (#3176)

This should solve the issue of undefined `this` when importing from an ES6 module.
I've put steps to reproduce in [this comment](https://github.com/axios/axios/issues/1861#issuecomment-669832435).

Co-authored-by: Jay <jasonsaayman@gmail.com>

webpack.config.js

@@ -11,7 +11,8 @@ function generateConfig(name) {
       filename: name + '.js',
       sourceMapFilename: name + '.map',
       library: 'axios',
-      libraryTarget: 'umd'
+      libraryTarget: 'umd',
+      globalObject: 'this'
     },
     node: {
       process: false