Modified the Modal component to allow modalRoot to also be the target…

[AJStacy]

… of scroll locking styles. I created a alt-modal-root demo page as well to see the changes in action.

Description

[Include a summary of the changes and the related issue.]

[Also include relevant motivation and context.]

How has this been tested?

[How did you test to verify your changes?]

[How can reviewers test these changes efficiently?]

[Check for unexpected visual regressions, see CONTRIBUTING.md for details.]

Documentation changes

[Do the changes include any API documentation changes?]

• Yes, this change contains documentation changes.
• No.

Related Links

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Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[just-boris]

As discussed in the linked issue, #423 (comment), this change is not needed.

Feel free to continue the discussion in the issue thread