[bpk-component-modal] Migrate customPropTypes to TypeScript

[Gert-Jan Vercauteren (gert-janvercauteren)]

Summary

• Migrates bpk-component-modal from JavaScript to TypeScript
• Replaces prop-types with TypeScript interfaces
• Converts defaultProps to ES6 destructure defaults
• Removes Flow type annotations
• Renames .js source/test/story files to .tsx/.ts

Closes #4750

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4763 to see this build running in a browser.

[skyscanner-backpack-bot]

	Warnings
⚠️	Package source files (e.g. packages/package-name/src/Component.js) were updated, but snapshots weren't. Have you checked that the tests still pass?

Browser support

If this is a visual change, make sure you've tested it in multiple browsers.

Generated by 🚫 dangerJS against 678cc3c

[Copilot]

Pull request overview

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Migrates bpk-component-modal away from Flow/prop-types toward TypeScript-based typing, aligning modal prop validation with the TS codebase.

Changes:

• Replaces Flow annotations with TypeScript type definitions for custom validators.
• Introduces ValidatorProps/Validator types and applies them to titlePropType and onClosePropType.

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[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4763 to see this build running in a browser.