This updates the block-handling logic in `Json::versions_from_content`
to naively pass the regex value when the block has two parameters. Up
to now, we have been ensuring that `regex` is not `nil` and this
makes sense with existing usage (the `Crate` strategy's default
block, formulae/cask `strategy` blocks). However, we need to allow a
`nil` `regex` value to make it possible to add an optional `regex`
parameter in the `Pypi::DEFAULT_BLOCK` Proc. This is necessary to
allow the `Pypi` strategy to work with an optional regex from a
`livecheck` block again [without creating an additional
`DEFAULT_BLOCK` variant with a regex parameter].
This setup mimics the `#parse_xml` method that was implemented in the
`Xml` strategy. Isolating the parsing code means that other strategies
can take only what they need from `Json` (i.e., it's not required for
them to use `Json#find_versions`).
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`).
