Add JSON lockfile schema and validation function

[jrdnbradford]

Here's my first go at issue #1877.

Why Use JSON Schema:

• Provides an easy, declarative way to document renv.lock
• Integrates with text editors to provide autocompletion, hover help, and validation
• Allows automated validation in CI, builds, tests, etc for R applications

General Decisions

• A relatively relaxed schema to prevent false negatives, with the option for users to pass their own schema that fits their personal requirements. Additional fields are allowed.
• Heavy on positive and negative testthat tests, so that we can verify updates to the schema
• The function wraps jsonvalidate::json_validate() and takes/passes many of its parameters there. I have not implemented all of them, but we can add if you’d like.
• Ease of use. Calling without parameters should cover most cases: renv::lockfile_validate()

Questions

• Do I have all the fields renv knows about?
• Do I have required set only for the fields that are absolutely necessary?
• I’ve used descriptions from renv documentation whenever possible, but had to make some inferences. I myself have never used anything that added Bioconductor, Python, or a few other fields. These should be checked.

[jrdnbradford]

Updates have been pushed.

Presuming this gets merged, feel free to @ me on issues, bugs, etc on this. Schemas and their validation is always a balancing act, and the R community at large may need different tradeoffs.

Closes #1877.

[kevinushey]

A huge thanks again for putting this together!