BridgeJS: Swift struct support

[krodak]

Introduction

This PR adds ability to export Swift structs to JS / TS using BridgeJS plugin.

Example

@JS struct Point {
    var x: Double
    var y: Double

    @JS init(x: Double, y: Double) {
        self.x = x
        self.y = y
    }
}

In JavaScript:

const point = exports.Point.init(10.0, 20.0);
console.log(point.x); // 10.0

TypeScript:

export interface Point {
    x: number;
    y: number;
}

export type Exports = {
    Point: {
        init(x: number, y: number): Point;
    }
}

Additional Changes

• refactored code around static methods / properties to keep it DRY after struct introduction
• some additional code organisation around code gen on Swift side

Testing

Added comprehensive snapshot and runtime tests for struct.

Documentation

Extended current documentation with new Exporting-Swift-Struct.md. It also covers differences between class and struct initialization.

Next steps

Will issue additional PRs with:

• struct benchmarks
• default value support for struct type

[kateinoigakukun]

Overall approach seems well thought out, let's incubate it on the main branch. Thank you for your huge effort!