refactor: use cross-fetch instead of node-fetch

Previously, we had Webpack's ProvidePlugin injecting `node-fetch` as a
polyfill into Node builds, and web builds just relied on the `fetch`
built into the browser (if it's there).

Now, we use `cross-fetch` everywhere, and as a ponyfill instead of a
polyfill. This broke the unit tests in `fetch-tool.js` because they were
overriding `global.fetch` to mock `node-fetch`. Now, those tests use
`tap.mock` to mock `cross-fetch`. Of course, `tap.mock` didn't exist in
the old version of `node-tap`, so I also upgraded that.

webpack.config.js

@@ -1,5 +1,4 @@
 const path = require('path');
-const {ProvidePlugin} = require('webpack');
 const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
 
 const base = {
@@ -65,11 +64,6 @@ module.exports = [
             'js-md5': true,
             'localforage': true,
             'text-encoding': true
-        },
-        plugins: [
-            new ProvidePlugin({
-                fetch: ['node-fetch', 'default']
-            })
-        ]
+        }
     })
 ];