[Shipping labels] Add package selection button for Woo Shipping

[rachelmcr]

Part of: #13550
⚠️ Depends on #14023 ⚠️

Description

This adds a package selection button ("Add a Package") to the new Woo Shipping label creation flow.

The button is in its own WooShippingPackageSelection view, to support the additional package UI that will be added in #13551 (showing e.g. the selected package instead of the button after selecting a package). That issue will also add the button action, to open the package selection UI.

This PR only contains view changes, so there are no view models or unit tests.

Testing information

1. Install and set up the Woo Shipping extension on your store.
2. Build and run the app with the revampedShippingLabelCreation feature flag enabled.
3. Create an order with the processing status and at least one physical product.
4. In the order details, select "Create Shipping Label."
5. Ensure the "Add a Package" button appears below the hazmat section in the shipping label creation screen.

Screenshots

Design:

Light	Dark

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr14024-09aa98b
	Version	20.5
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	09aa98b
	App Center Build	WooCommerce - Prototype Builds #10966

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