Improve a11y for template counter component

[geoffrich]

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This PR includes the following a11y improvements for the default template:

• The Counter component renders the next number out-of-view so it can slide the next number in. This number was still visible to assistive tech such as screen readers, which read out both numbers as if they were one. For example, NVDA read out "10" in the starting state, when only "0" was visible on screen. Adding the aria-hidden attribute to the next number hides it from screen readers so they only read out the visible number.
• I also added aria-hidden to the SVGs in the + and - buttons. The aria-label on the buttons overrides the label on the SVGs, and they don't need to be read out to screen readers.

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[Rich-Harris]

thank you so much!