Add `init(unsafelyWrapping:JSObject)` on `@JS struct`

[MaxDesiatov]

The goal is to support the following code

import JavaScriptKit

@JS protocol JSON {
    func parse(_ message: String) throws(JSException) -> JSObject
    func serialize(_ message: JSObject) throws(JSException) -> String
}

@JSGetter(from: .global) var json: JSON

@JS struct ShoppingCart {
    var x: Int

    @JS public func toJSON() throws(JSException) -> String {
        json.serialize(self.jsObject)
    }

    @JS static public func fromJSON(_ string: String) throws(JSException) -> ShoppingCart {
        self.init(unsafelyWrapping: json.parse(string))
    }
}

There's an opinion that such struct should follow value semantics, for which it has to copy its property values out to Wasm linear memory out of JS. To control copying strategies we might consider adding an argument to @JS controlling this, i.e. @JS(copy: .lazy), @JS(copy: .eager).

In the long term we should also support @JS struct Foo: ~Copyable that avoids copies altogether and always gives access to backing JS object reference.