Allow static builds to use Remote Functions

[theetrain]

Describe the problem

Remote functions query, form, and command are only available when shipping SvelteKit with a server, such as with serverless or Node.js adapters. Anyone shipping desktop applications cannot leverage Remote Functions today.

Describe the proposed solution

Provide an opt-in configuration that allows static builds to consume query, form, and command functions client-side so long as they point to remote functions deployed elsewhere. As an idea, we could add a config option such as kit.remoteFunctions.origin for developers to set their remote server, such as https://api.example.com or an environment variable. Absence of this configuration should fail the build for any prerendered or static pages consuming the above remote function types.

This unlocks being able to maintain a single codebase that can use Remote Functions and conditionally deploy as a native (Capacitor, Electron, Tauri, Wails, etc) app with adapter-static, or a website using any server-like adapter such as adapter-node.

The challenge with ensuring compatibility between the native builds and their corresponding remote site is avoiding drift between breaking changes in remote function; it's very possible for the remote application to ship before users update their local/native apps. Some mitigation strategies could be:

1. Check for version when remote functions return a 'not found' error and provide a signal. That way in userland we can show a notice to the end user to refresh or update their application.
2. Application maintainers could version their remote functions, either in the directory structure such as shipping a v1 and v2 file to coexist until all consumers upgrade (src/lib/v1/todos.remote.ts, src/lib/v2/todos.remote.ts)
3. Function exports can be versioned in their name (const getTodos = query(/* ... */), const getTodos_v2 = query(/* ... */)).

Alternatives considered

Adopt a third-party querying library.

Importance

nice to have

Additional Information

Related comment: #13897 (reply in thread)

[gyurielf]

That would be really cool!
This means it will work with SPA as well, right ? :))