SsrRequest
One page being rendered: its URL, the server component trees rendered so far (Flight's text, by component and
props), and how to render one, passing its text to write as it's ready.
export interface SsrRequest { url: URL; cache: Map<string, unknown>; trees: Record<string, string>; render(hash: string, body: string, write: (text: string) => void): Promise<void>; }