[bpk-component-scrollable-calendar] Migrate remaining JS file to TypeScript

[Gert-Jan Vercauteren (gert-janvercauteren)]

Summary

• Migrates bpk-component-scrollable-calendar 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 #4752

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Pull request overview

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Migrates bpk-component-scrollable-calendar remaining JavaScript artifacts toward TypeScript by adding type annotations and replacing runtime prop validation with static typing.

Changes:

• Adds TypeScript type annotations to date formatter helpers.
• Introduces WeekDay typing for exported weekday arrays.

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[skyscanner-backpack-bot]

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	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 b28eb86

[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4765 to see this build running in a browser.