Add source code editor

[justinmacorin]

Implement a source code editor instead of using a HTML contenteditable DIV for the model editor.

Reasoning: Validating, writing, and reading JSON is challenging with no styling. A source code editor can help with this. In addition, it can be used for future source-code related features - like code execution, source code modification, etc.

Files:

• frontend/pages/models.tsx (modify the contenteditable pre tag)
• NEW CodeEditor.tsx file

Here is an example of a good code editor: https://microsoft.github.io/monaco-editor/