This refactors verbose code in the `Sparkle` strategy where we access
element text into a reusable `Xml#element_text` method, replacing
chained calls like `item.elements["title"]&.text&.strip&.presence`
with `Xml.element_text(item, "title")`.
`#element_text` is only used to retrieve the text of a child element
in the `Sparkle` strategy but it can also retrieve the text from the
provided element if the `child_path` argument is omitted (i.e.,
`Xml.element_text(item)`). This will allow us to also avoid similar
calls like `item.text.strip.presence` in the future.
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