[kbn-code-owners] General improvements (#204023)

## Summary
The following improvements have been made:
- Added `--json` flag to CLI command to output result as JSON
- Terminology updated to more accurately reflect object contents
- Code owner teams are always returned as an array
- Search path validation (is under repo root, exists)
- Proper handling of inline comments
- Better logging for `scripts/check_ftr_code_owners.js`

Existing usage of the `@kbn/code-owners` package has been updated
accordingly, without modifying outcomes.

scripts/get_owners_for_file.js

@@ -8,4 +8,4 @@
  */
 
 require('../src/setup_node_env');
-require('@kbn/code-owners').runGetOwnersForFileCli();
+void require('@kbn/code-owners').cli.findCodeOwnersForPath();