BridgeJS: Change @JS init to generate static init() methods instead of constructor in JS

[kateinoigakukun]

Changes @JS init to generate static init() methods instead of direct constructors to separate two distinct Swift class instantiation patterns.

Problem

Previously, new SwiftClass() was used for both:

1. Calling Swift @JS init constructors
2. Lifting Swift class instances returned from exposed Swift functions (Swift side returns pointer, JS glue code lifts it into wrapper object)

These two operations should not share the same API as they have fundamentally different semantics.

Solution

• @JS init now generates static init() methods for Swift constructor calls
• new SwiftClass() is reserved for lifting Swift instances returned from Swift functions
• Fixed handling of Swift classes without @JS init constructors

Changes on generated interface

// Before
export type Exports = {
    Greeter: {
        new(name: string): Greeter;
    }
}
const { exports } = await instantiate(...)
const greeter = new exports.Greeter("world");

// After
export type Exports = {
    Greeter: {
        init(name: string): Greeter;
    }
}
const { exports } = await instantiate(...)
const greeter = exports.Greeter.init("world");