[Flyout System] improve size validation flexibility and bidirectionality

[tsullivan]

Summary

• Allow main flyouts to use custom size values (e.g., '400px') while child flyouts must use named sizes
• Update 'l' size validation to be bidirectional: either flyout can be 'l' if the other is 'fill'
• Change the default size of child flyouts to be s so that default sizing for both parent and child results in a valid combination
• Refactor validation logic to use array methods for cleaner, more concise code
• Update all tests to reflect new validation rules
• Update documentation and storybook with correct size combination rules
• Add 'l' and '400px' options to flyout manager storybook controls

Why are we making this change?

We have to allow flexible sizing options to support existing use cases in Kibana and provide an easy migration path for those flyouts to adopt the Flyout System.

Screenshots #

Impact to users

Impact to devs: easier migration path to adopt existing flyouts into the Flyout System.

QA

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General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in both MacOS and Windows high contrast modes
    • (emulate forced colors if you do not have access to a Windows machine.)
  • 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
  • Updated visual regression 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
  • If applicable, file an issue to update EUI's Figma library with any corresponding UI changes. (This is an internal repo, if you are external to Elastic, ask a maintainer to submit this request)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 677d2c8
• Documentation website
• Storybook

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 677d2c8

[tkajtoch]

LGTM