Finalize JS API for startServer()

[FredKSchott]

Changes

• Tackles a large chunk of Finalize SSR & JS API #1339
• Finalizes our startServer() JS API (no longer __unstable_startServer(), now covered by semver)
• Previously, the functions returned by the startServer() API had been implemented as JS wrappers around HTTP calls to the dev server. To build a better JS API, we flipped it: the dev server is now an HTTP layer on top of this same exported JS API.

New API

export function startServer(commandOptions: CommandOptions): Promise<ServerResult>;
export interface ServerResult {
    loadUrl: (reqUrl: string, options?: {isSSR: boolean; allowStale: boolean}) => Promise<LoadResult>;
    handleRequest: (req: http.IncomingMessage, res: http.ServerResponse, options?: {handleError?: boolean}) => Promise<void>;
    sendResponseFile: typeof sendResponseFile;
    sendResponseError: typeof sendResponseError;
    shutdown(): Promise<void>;
}

Testing

• Still TODO (this API is still experimental, undocumented, and not yet supported. Will tackle in follow up PR!)

Docs

• Still TODO

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[orta]

General note: if you make this /** @deprecated Promoted to startServer() **/ then it shows up in the editor with a line through

[FredKSchott]

great tip, thanks!