replace default value int index.d.ts with optional parameters

[lysz210]

When I first wrote the constructor declarations with defaults, VSCode didn't rais any errors, but after reopen it and declaring interfaces it raised:
A parameter initializer is only allowed in a function or constructor implementation.ts(2371)
VSCode isn't consistent with this notification.

class Point2D {
  // constructor (x: number = 0, y: number = 0)
  // replace with
  constructor (x?: number, y?: number)
}
// all this initialization are good
Point2D p0 = new Point2D() // x and y initialization are handled by the real implementation
Point2D p1 = new Point2D(1) // y initialization is handled by the real implementation
Point2D p2 = new Point2D(1, 1)