This adds a generic `Json` strategy to livecheck that requires a
`strategy` block to operate. This is primarily intended as a
replacement for existing `strategy` blocks in formulae/casks that
use `JSON#parse`, as it allows us to internalize/standardize that
boilerplate while improving error-handling.
Additionally, future strategies that parse JSON data can use the
`Json#find_versions` method instead of having to reinvent the wheel
(similar to how we currently have a number of strategies that
leverage `PageMatch#find_versions`).
