Add EuiWindowProvider for multi-window support

[stil]

Summary

Closes #7778

The main idea in this PR is to enable multi-windowed React development with EUI, for example in Electron.
The change is purely bug-fixing and does not introduce any UI changes.

I want to enable multi-windowed support by using React Portals to render into different DOM windows.
The problem is that, in many places, EUI operates on global window or document object which I aim to fix.

It's not a completely novel idea, something similar can be found in FluentUI: https://github.com/microsoft/fluentui/tree/master/packages/react-window-provider

New <EuiWindowProvider> component is exposing current window object for its children elements.

I'm marking it as draft so early-on you can have a look what's inside and perhaps immediately tell that it's not going to be merged, then I'll continue work internally.

The change is completely backwards compatible and does not introduce any behavior or UI changes.

Remaining work:

• See why outside click does not fire in child windows (Working with components portalled to child DOM windows theKashey/react-focus-on#88).
• Do additional testing of different kind of behaviors in my client application.

The most important is that with these changes Flyouts work in child windows!

Let me know wheter this is something you'd consider at all for merging (assuming that all potential issues with PR are fixed).

QA

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
  • Props have proper autodocs (using @default if default values are missing) and playground toggles
  • Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
• Release checklist
  • A changelog entry exists and is marked appropriately.
  • If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist
  • Updated the Figma library counterpart

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[github-actions]

👋 Since this is a community submitted pull request, a Buildkite build has not been started automatically. Would an Elastic organization member please verify the contents of this pull request and kick off a build manually?

[github-actions]

👋 Hey there. This PR hasn't had any activity for 90 days. We'll automatically close it if that trend continues for another week. If you feel this issue is still valid and needs attention please let us know with a comment.

[github-actions]

❌ We're automatically closing this PR due to lack of activity. Please comment if you feel this was done in error.

[stil]

Please reopen - I'm still working on fixes and now it's in better state than before with many bugs fixed.

[cee-chen]

Re-opening! Apologies @stil, we hadn't even realized this PR was in the queue - we don't get automatically pinged/notified for draft PRs unless they open.

[github-actions]

👋 Since this is a community submitted pull request, a Buildkite build has not been started automatically. Would an Elastic organization member please verify the contents of this pull request and kick off a build manually?

[cee-chen]

buildkite test this

[cee-chen]

As a heads up you will need to sign the Elastic CLA with the email address associated with the git commits (use git log to see that email address).

[elasticmachine]

💚 Build Succeeded

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• Commit: 3b73f6c