feat(core/modal): migrate to shadow dom

[danielleroux]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
• Linting (npm lint) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

GitHub Issue Number: #511 #507

What is the new behavior?

• Replace custom modal implementation with HTMLDialog
• Add ix-modal-header, ix-modal-content, ix-model-footer to handle layouting more structured
• Add showMessage function to show default messages (into, success, warning, error, question)
• Add showModalLoading to loading spinner as modal
• Enhance documentation regarding new functionality
• Add JSDocs to all empty Prop/Event decorators

Does this introduce a breaking change?

• Yes, see BREAKING_CHANGES.md
• No

[nuke-ellington]

Some minor tings

[nuke-ellington]

😍

[nuke-ellington]

lgtm