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Geometry Nodes: Add fields version of material nodes

This commit moves the old material nodes to a "legacy" folder and adds
versions of the nodes that work with fields.

The "Select by Material" node is a field node now, so it doesn't have
a geometry output. This is an improvement because there are fewer links
to connect, and it's more easily usable in different situations.
It's also called "Material Selection", since it's more of an input
than an action now.

It's sometimes necessary to use the attribute capture node to get a
more predictable interpolation to mesh faces. This is because the
selection field input is always evaluated on the face domain, so
attribute inputs are interpolated before they are booleans, so they
cannot use the new interpolations from rB5841f8656d9580d7b9.

Differential Revision: https://developer.blender.org/D12456