[bpk-component-calendar] Migrate remaining JS files to TypeScript

[Gert-Jan Vercauteren (gert-janvercauteren)]

Summary

• Migrates bpk-component-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 #4751

[Copilot]

Pull request overview

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Migrates remaining bpk-component-calendar JavaScript code toward TypeScript by introducing typed props/utilities and removing prop-types usage in story components.

Changes:

• Added TypeScript type annotations and literal typing (as const) to legacy prop-type helpers.
• Replaced prop-types in story components with TypeScript interfaces and destructured defaults.
• Removed a @ts-expect-error suppression on the custom-proptypes-legacy import.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/backpack-web/src/bpk-component-calendar/src/custom-proptypes-legacy.js	Adds TS types/exports and narrows selection constants.
packages/backpack-web/src/bpk-component-calendar/src/BpkCalendar.stories-test-utils.js	Adds TS annotations for date formatting helpers and weekday arrays.
packages/backpack-web/src/bpk-component-calendar/src/BpkCalendar.stories-components.js	Migrates story components away from prop-types and adds TS-typed props/state.
packages/backpack-web/src/bpk-component-calendar/index.ts	Removes suppression for importing legacy prop-types module.

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[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/4764 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

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