This adds a generic `Xml` strategy to livecheck that requires a
`strategy` block to operate. The XML-parsing code is taken from the
existing approach in the `Sparkle` strategy. As such, `Sparkle` has
been updated to use the `Xml#parse_xml` method instead.
Unlike the `Json` strategy, we don't currently have any `strategy`
blocks in first-party taps that manually parse XML. However, we had a
user request support for something like this and I was already working
on an `Xml` strategy (as a way of extracting the XML-parsing code
from `Sparkle` into something general-purpose), so here we are.
Future strategies that parse simple XML data can potentially use the
`Xml#find_versions` method (similar to how we have strategies that
leverage `PageMatch#find_versions`) instead of having to implement
something bespoke like `Sparkle`.
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