Fix flash of content on initialization

[colsondonohue]

Does two things to eliminate the flash, when using the ReactCodeMirror component:

1. Eliminate one initial re-render from calling setContainer in an effect when the container prop changes

• previously, this resulted in setContainer(null) -> re-render -> setContainer(el) -> re-render -> [now component can initialize]
• now, the first effect invocation no-ops, since the ref value is not truthy, then the second time no state is updated, since props.container is referentially equal to container, set by the ref callback when the div mounted

2. Eliminate one paint by running initialization in a layout effect, which forces the second re-render to occur synchronously before paint

So the new set of render cycles on mount is:
mount -> setContainer in callback ref -> re-render -> layout effect to initialize editor state -> re-render -> paint

It won't eliminate flashing when users choose to use useCodeMirror directly, unless they are also calling setContainer in a callback ref on their parent element.

Ideally, I think useCodeMirror would not take a container prop at all, and would instead do all of the state/view initialization logic in setContainer, which would be a callback ref. However, this is a breaking change, and I don't want to remove all of the synchronization logic in effects which enables passing a different container.