feat(diff): Better diff of random objects

[RomainMuller]

Use a unified diff format to render differences in arbitrary values,
making it easier to understand what is changing in possibly large JSON
structures, for example.

The number of context lines used when rendering the JSON differences
can be customized using the --context-lines option of cdk diff,
which has a default value of 3.

---

Pull Request Checklist

• Testing
  • Unit test added
  • CLI change?: manually run integration tests and paste output as a PR comment
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[eladb]

Can you upload a screen shot?

[RomainMuller]

Screenshot using the default context of 5 lines (can be customized all the way down to zero!)

[eladb]

Please make sure to run toolkit integ tests

[RomainMuller]

Integration tests were successful!

[rix0rrr]

Can we reduce the default context some? I'd prefer something like 3. The diff takes up a lot of vertical space this way.

[RomainMuller]

@rix0rrr - yeah that'll match the defaults of most diff command implementations, so I'm cool with that.