Add cursor to the FormattableAmountTextField

[toupper]

The FormattableAmountTextField that we use in Custom Amounts, Order Shipping Methods and Cash Payments doesn't have a cursor, which makes difficult it to the user to enter their desired value.
From here:

I included the cursor in all my designs for this text field style. It’s the most commonly used element of all text fields to indicate not just that the field is focused, but also where the user should expect to see their typing appear. I’ve been meaning to raise it in my design dusting posts soon.

ichqhxDjbmZZmvJaJkGhkj-fi-698_29716 (Link to Figma designs for custom amounts)

As for focusing the field, the border below the text area is supposed to be the bottom of the field, so my first thought is to try making that border bold and purple. This is probably a good idea even if we show the cursor.

[dangermattic]

Thanks for reporting! 👍

[bozidarsevo]

Since #14030 added the border to show that the field is in focus, should we still change the border line below the field?

Also regarding the cursor, since we use "Hidden input text field" in FormattableAmountTextField.swift and there is no way of getting the position of the cursor in the TextField, I suppose we would need to rewrite this ``FormattableAmountTextField` to either use UIKit component or some other way to get the cursor position.

What do you think?

[toupper]

Since #14030 added the border to show that the field is in focus, should we still change the border line below the field?

Good questions @bozidarsevo :) Yes, this task is about adding the cursor and updating the border to the design above, showing just the line below the field.

Also regarding the cursor, since we use "Hidden input text field" in FormattableAmountTextField.swift and there is no way of getting the position of the cursor in the TextField, I suppose we would need to rewrite this ``FormattableAmountTextField` to either use UIKit component or some other way to get the cursor position.

Yes, we would have to find a way to update FormattableAmountTextField to properly show the cursor. But in any case, let's use the FormattableAmountTextField as it is now and postpone its refactor until we have more time to tackle that.