Add support for running an srcDoc style iFrame

[glendaviesnz]

As noted here, there are some instances where iFraming react-easy-crop causes the styles and events to be lost as they are always attached to the global window and document.

This PR changes the Cropper componentDidMount method to instead get the document and window references from the current containerRef if it is defined.

The PR also adds an iFramed version of the component to the example page to allow for testing. This can be accessed via http://localhost:3001/?iframed=true.

[codesandbox-ci]

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[glendaviesnz]

@ValentinH have just left this as draft for now so you can check if you are happy with the general approach or not - if you are I will sort the remaining type issues and add some extra tests before getting a full review.

[glendaviesnz]

@ValentinH I think it should be good to go. All the existing e2e tests pass with the change in place, and I added one additional test to make sure the styles get applied to the iframe doc - let me know if you think we need any additional tests on top of this.

[ValentinH]

All good!

[github-actions]

🚀 PR was released in v4.5.1 🚀

[glendaviesnz]

Big thanks for the quick turnaround on this @ValentinH!