Since we use `REXML::Document` in the type signature for `#parse_xml`,
we can encounter an `uninitialized constant
Homebrew::Livecheck::Strategy::Xml::REXML` error in strategies like
`Sparkle` that use `Xml#parse_xml` internally when the Sorbet runtime
is used. Moving the related require outside of the `#parse_xml` method
and into the `Xml` strategy proper resolves this issue.
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`.
