Add json validator component, to be used by upcoming manifest.json validator.
Add json validator tests.
Add custom validator for "manifest.json" files.
Code cleanup.
Rename "ErrorSink" to "ErrorCollector"
Rename "emitMessage" to "addMessage".
Address code review feedback.
Merge pull request #3191 from rpaquay/json-validator-2

Inittial end-2-end implementation of a builder for "manifest.json" files.