Make Gosling.js monorepo

[sehilyi]

Related comments

• In both the editor and the document, examples can be consistent using, e.g., gosling.js/examples
• gosling-theme can be moved to here #860
• We need to consider moving examples in gosling-lang/gosling-react-example to this repo under src/examples

One thing to consider would be moving an examples/ folder to gosling.js and managing each of the examples (rather than every app needing react-dom-router) by @manzt