js,parser: Allow declaring methods on JS interfaces for easier FFI

[playXE]

This PR allows to declare methods on JS interfaces, this allows to create high-level wrappers without wrapping interfaces in another V struct. For example:

pub interface JS.CanvasRenderingContext2D {
	setLineDash(JS.Array)
}

pub fn (ctx JS.CanvasRenderingContext2D) set_line_dash(arr []f64) {
	tmp := JS.Array.prototype.constructor()

	for x in arr {
		tmp.push(JS.Number(x))
	}
	ctx.setLineDash(tmp)
}