Add support for C++23 multidimensional subscript operator

Tyler Venessrequested to merge
calcmogul/eigen:add-support-for-cpp23-multidimensional-subscript-operator into master

I'm not sure where to put tests for this, assuming they're needed. They also wouldn't run in CI anyway since CI only exercises the C++17 codepaths.

Edited by Tyler Veness

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