Setters should return their argument

[ChayimFriedman2]

In Wren, assignment returns its value, like in C:

var a
System.print(a = 5) // 5

When implementing setters, we need to maintain this behavior:

// Not good
class C {
  static a=(value) {
    __a = value
  }
}
System.print(C.a = 5) // null

// Good
class C {
  static a=(value) { __a = value }
}
System.print(C.a = 5) // 5

// Also good
class C {
  static a=(value) {
    __a = value
    return value
  }
}
System.print(C.a = 5) // 5

Generally, the CLI do that, except in one foreign setter - Stdin.isRaw=(_):

https://github.com/wren-lang/wren-cli/blob/b82cf5a2e21ef54a8ac8347b27899edfeeffa724/src/module/io.c#L509-L525

Instead we should do:

void stdinIsRawSet(WrenVM* vm)
{
  initStdin();
  
  isStdinRaw = wrenGetSlotBool(vm, 1);
  
  if (uv_guess_handle(stdinDescriptor) == UV_TTY)
  {
    uv_tty_t* handle = (uv_tty_t*)stdinStream;
    uv_tty_set_mode(handle, isStdinRaw ? UV_TTY_MODE_RAW : UV_TTY_MODE_NORMAL);
  }
  else
  {
    // Can't set raw mode when not talking to a TTY.
    // TODO: Make this a runtime error?
  }

  wrenEnsureSlots(vm, 1); // Is that required? Probably not
  wrenSetSlotBool(vm, 0, isStdinRaw);
}