CSM: Remove array allocation in getBreaks, getExtendedBreaks

[gkjohnson]

Follow up to #18724.

Modifies the getBreaks function and getExtendedBreaks function to reuse existing arrays rather than creating new ones every call to prevent garbage collection.

This also implies a small API change to customSplitsCallback, which now needs to modify an existing array in target rather than return a new one:

customSplitsCallback(
   cascades: Number,
   near: Number,
   far: Number,
   target: Array<Number>
): void

Temporary Live Link:
https://raw.githack.com/gkjohnson/three.js/csm-breaks-cache-arrays/examples/webgl_shadowmap_csm.html

@vhawk

Edit: Looks like the CI failure is from css3d_youtube diff again

ERROR! Diff wrong in 0.254 of pixels in file: css3d_youtube

[mrdoob]

Thanks!