BufferGeometryUtils: allow tree-shaking

[marcofugaro]

Related issue: Similar to #21651

Description
This allows BufferGeometryUtils to be tree-shakeable, so users don't have to include every function if they use only one of them.

The only difference is that

import { BufferGeometryUtils } from '...';

becomes

import * as BufferGeometryUtils from '...';

[mrdoob]

Thanks!

[mrdoob]

/fyi @agviegas

[marcofugaro]

I'll convert other util files as well.
I think it's better if we use this pattern from now on, instead of the class + static methods.

[mrdoob]

Seems like the the current babel script can't handle this pattern:

[marcofugaro]

I'll do a PR!

[mrdoob]

Hmm, there is still something not quite right:

3614903#diff-0d07c3fa7b8b29a449a8f5f35099c5d9994726a038d441256103891cb39e899fR28

[marcofugaro]

@mrdoob thanks for the report, fixed in #22306!