Add JSON output for diagnostics

[funwithcthulhu]

Adds doctor check --json for machine-readable diagnostic output.

Summary:

• adds JSON rendering for existing diagnostics;
• preserves the default text output;
• preserves current exit-code semantics;
• adds tests around JSON output and opam environment parsing;
• updates README and changelog for 0.2.0.

Validation:

• dune build
• dune runtest
• opam lint doctor.opam