[json] open definition in JSON document to navigate to the schema definition

- VS Code Version: Version: 1.62.3 (Universal) - OS: Darwin x64 19.6.0 Steps to Reproduce: [VSC bug 1.zip](https://github.com/microsoft/vscode/files/7663115/VSC.bug.1.zip) 1. open bug1.json from attached minimal project 2. position cursor over field1 (line 2, column 7) 3. Go to Definition Expected: jump to bug1.schema.json, line 5, column 5. Actual: nothing happens. Maybe I expect too much? I'd like to edit both example and its specification file in parallel and am very interested in cross-navigation