BridgeJS: Support generic functions on exported Swift APIs

[krodak]

Overview

Adds generic exported Swift functions and methods. Swift-owned scene graphs, document models, and registries can expose typed operations without a separate binding for every model type. JavaScript callers select concrete Swift types with generated BridgeTypes tokens.

Example

A scene maintained by Swift can expose checked lookups and collections:

@JS protocol SceneNode: BridgedSwiftGenericBridgeable {
    var id: String { get }
}

@JS final class Scene {
    private var nodes: [String: any SceneNode]

    init(nodes: [String: any SceneNode]) { self.nodes = nodes }

    @JS func find<T: SceneNode>(_ id: String) -> T? {
        nodes[id] as? T
    }

    @JS func all<T: SceneNode>() -> [T] {
        nodes.values.compactMap { $0 as? T }
    }
}

The application supplies scene; Camera and Light are exported final classes conforming to SceneNode:

import { BridgeTypes } from "./bridge-js.js";

const camera = scene.find("main-camera", BridgeTypes.Camera); // Camera | null
const lights = scene.all(BridgeTypes.Light); // Light[]
const wrongType = scene.find("main-camera", BridgeTypes.Light); // null

Swift checks the stored object against the requested type. The token also determines the TypeScript return type, avoiding unchecked casts or per-type methods such as findCamera and findLight.

Supported Forms

• Supports T, [T], T?, and [String: T], mixed with concrete parameters and results.
• Supports async/throwing exports and generic callback parameters, including escaping and async callbacks.
• Preserves @JS protocol constraints in TypeScript and rejects unknown or non-conforming tokens before lowering arguments.
• Exported generic initializers, generic nominal types, and Embedded Swift exports remain unsupported. Imported generic @JSClass initializers are unchanged.

Test Plan

• BridgeJS codegen, diagnostics, and snapshot tests across SwiftSyntax 600-603.
• npm run check:bridgejs-dts.
• ./Utilities/bridge-js-generate.sh.
• make unittest SWIFT_SDK_ID=swift-6.3.1-RELEASE_wasm BUILD_SYSTEM=native.