[Shipping Lines] Update styles on Shipping screen in order form

[rachelmcr]

Part of: #12578

Why

This updates the styles on the Add/Edit Shipping screen in the order form, when the enhancingOrderShippingLines feature flag is enabled.

How

• Adds a new ShippingLineSelectionDetails view and view model. I decided to use a new view because the style changes also meant some significant changes in the view's logic and data handling. This will make it easier to remove the old view when we release these changes.
• Migrates the unit tests from ShippingLineDetailsViewModel to test the same functionality in ShippingLineSelectionDetailsViewModel.
• Adds the new view model to EditableOrderViewModel.
• Updates OrderForm to navigate to the new view when the feature flag is enabled.
• The new view uses FormattableAmountTextField for editing the shipping amount. I updated that view and its view model:
  • Add support for different font sizes.
  • Update the placeholder to use the placeholder text color.
  • Optionally allow negative numbers (and provide a keyboard for entering negative numbers if allowed).
• Updates UI tests for adding shipping on the new screen.

Testing instructions

1. Build and run the app.
2. Create a new order.
3. Add a product to the order.
4. Tap the "Add Shipping" button to open the shipping details screen.
5. Confirm you can add a shipping amount (positive or negative) and name.
6. Add the shipping to the order.
7. In the order total section, select the shipping row to edit it.
8. Confirm you can edit the shipping amount and name.
9. Confirm you can remove the shipping from the order.

Screenshots

Existing styles (feature flag disabled)	New styles (feature flag enabled)

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

[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	pr12621-9bb74ec
	Version	18.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	9bb74ec4272fc000d0b3b1097f2a841037cfb19a
	App Center Build	WooCommerce - Prototype Builds #8911

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[dangermattic]

	2 Warnings
:warning:	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.
:warning:	This PR is assigned to the milestone 18.5. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by :no_entry_sign: Danger

[rachelmcr]

@woocommerce/mobile-ui-testing-squad I'm going to go ahead and merge these changes. Please let me know if you have any concerns about the UI test changes (https://github.com/woocommerce/woocommerce-ios/pull/12621/commits/ccbfabe994977dabc16906db739994a5ecc3b0dd) and I can address them in a followup PR.