Implement fallback children

When you pass a fallback to renderIntoDocument it will emit the fallback if the shell of the primary children errors. It is possible for the fallback to also error in the shell in which case the result is a fatalError as if you did not provide a fallback.

Included in this change is a refactoring of how preamble and postamble work. They are now interned within the resposneState where we can have more careful control over what is flushed and when.

There is also a new concept of an early preamble. the early preamble is the html and head tags plus any resources that can flush (most things plus stylesheets up to the first precedence)

packages/react-dom-bindings/src/server/ReactDOMFloatServer.js

@@ -129,6 +129,8 @@ export type Resources = {
   preconnects: Set<LinkResource>,
   fontPreloads: Set<PreloadResource>,
   // usedImagePreloads: Set<PreloadResource>,
+  firstPrecedence: string,
+  firstPrecedenceFlushed: boolean,
   precedences: Map<string, Set<StyleResource>>,
   usedStylePreloads: Set<PreloadResource>,
   scripts: Set<ScriptResource>,
@@ -161,6 +163,8 @@ export function createResources(): Resources {
     preconnects: new Set(),
     fontPreloads: new Set(),
     // usedImagePreloads: new Set(),
+    firstPrecedence: '',
+    firstPrecedenceFlushed: false,
     precedences: new Map(),
     usedStylePreloads: new Set(),
     scripts: new Set(),
@@ -485,14 +489,17 @@ function createStyleResource(
       );
     }
   }
-  const {stylesMap, preloadsMap, precedences} = resources;
+  const {stylesMap, preloadsMap, precedences, firstPrecedence} = resources;
 
   // If this is the first time we've seen this precedence we encode it's position in our set even though
   // we don't add the resource to this set yet
   let precedenceSet = precedences.get(precedence);
   if (!precedenceSet) {
     precedenceSet = new Set();
     precedences.set(precedence, precedenceSet);
+    if (!firstPrecedence) {
+      resources.firstPrecedence = precedence;
+    }
   }
 
   let hint = preloadsMap.get(href);