fix: Add defaultContent to JSONEditor

[tylerlaws0n]

This will allow react or other frameworks to initialize the editor with content and still recieve
updated content through onChange, but not continue to control the value as it is changed.

Relates to #23

[tylerlaws0n]

@josdejong would really appreciate your feedback on this when you get a chance - Thank you!

[josdejong]

Why isn't the current content property sufficient for this? You decide what you pass there, and whether you want to update this property or not.

[josdejong]

I do like the "build:watch": "npm run package -- --watch" that you added 😁

[tylerlaws0n]

Very good point with regards to the value. What I found is probably better for this situation is to edit the sandbox to accept the prop on the react side, then have it not update, leaving the editor uncontrolled.

https://codesandbox.io/s/svelte-jsoneditor-react-forked-td9to