Select one default shipping option

## Problem

In dark mode while doing a checkout on store.elementary.io it looked like a bullet point I kept on getting error message as "Please fill out the form". (screenshot below)

The reason was that I did not select the only shipping option available, as it did not look like a radio button.


## Proposal

If there are no other options in a radio button scenario only one should be checked, and in case of multiple first one can be a good option to with.

Quick browsing on the code shows the fix may include adding `checked: true` [here](https://github.com/elementary/store/blob/9538e2f5a8dd3297ba38fbf0ed8b708bd0bd8f36/lib/store_web/templates/checkout/shipping.html.leex#L12), but I don't have much experience with Elixir so cannot be sure.

## Prior Art

I could not find any examples.