Examples: Allow tree-shaking for curves

[marcofugaro]

Related issue: -

Description

Remove the unnecessary Curves object to allow tree-shaking for curves.

The users that imported curves like this:

import { Curves } from 'three/examples/jsm/curves/CurveExtras.js';

will now import them like this:

import * as Curves from 'three/examples/jsm/curves/CurveExtras.js';

[mrdoob]

Thanks!