Json: Add #parse_json method and tidy up tests

[samford]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

After working on the Xml strategy, I came back to the Json strategy to make a few changes to better align the two. For example, the Xml strategy provides a #parse_xml method that only includes the parsing logic, so other strategies can selectively use this method if that's all they need (instead of Xml#find_versions). This PR modifies the Json strategy to provide a similar Json#parse_json method.

This also renames the simple_content* variables in json_spec to content_simple*, to align the variable name with the naming convention used in the Xml strategy (e.g., content_version_text, content_version_attr, content_simple, etc.). This doesn't make a functional difference but is a little more tidy/predictable.

I've also tweaked a few tests to use content_simple in place of content in tests where the content isn't actually used (aligning with similar Xml tests). It doesn't make a difference other than passing a smaller string as the argument.

[BrewTestBot]

Review period will end on 2023-03-02 at 23:02:52 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @samford!