Use mergeGeometries instead of mergeBufferGeometries

[mtsmfm]

Thank you for maintaining awesome library.

Since r151 mergeGeometries is preferrable.

mrdoob/three.js#25652

I checked past PR which addressed similar deprecation warn #230
I believe this library encourages users to update three.js version so I've updated package.json instead of keeping backward compatibility like the following:

import BufferGeometryUtils from 'three/examples/jsm/utils/BufferGeometryUtils.js';

const mergeGeometries =
	BufferGeometryUtils.mergeGeometries ||
	BufferGeometryUtils.mergeBufferGeometries;

Please let me know if you have a concern.