Remove the global requirement of this module

[ScopeyNZ]

Currently this module requires consuming packages to add something like this to their webpack config:

	// ...
    entry: {
      bundle: `${PATHS.SRC}/bundles/bundle.js`,
      injector: `${PATHS.MODULES}/@silverstripe/react-injector/dist/injector.js`,
    },
    externals: {
      'lib/Injector': 'Injector',
    },
	// ...

Only one consumer needs to define the entry and actually include the stand-alone injector.js file. The others have to include the externals config and just hope the API is there in globals.

This can potentially be improved by defining a global store in the module. This could look like this:

// configure Injector before use
Injector.configure({
  store: () => window.ss.injector; // Default = window.injectorStore (?)
});
// Use of injector with multiple stores
Injector.component.register('MyComponent', MyComponent, { store: window.vendorInjectorStore });

This would allow use as a more standard NPM module, no externals config required. Duplication of the injector code in different bundles would be acceptable.