Make QQuickWebEngineScript a publicly exported class

Now the users can create user scripts when they are working
with QQuickWebEngineProfiles in C++.

Change-Id: I6ada78b1fc44c3f93589eb468578c330a1be9c6f
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>