This adds a generic `Yaml` strategy to livecheck that requires a
`strategy` block to operate. The YAML-parsing code is taken from the
existing approach in the `ElectronBuilder` strategy.
We don't currently have any `strategy` blocks in first-party taps
that manually parse YAML. However, creating a generic `Yaml` strategy
allows us to simplify `ElectronBuilder` (and any future strategy
that works with YAML) while making it easy to create custom `Yaml`
`strategy` blocks in formulae/casks as needed.
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