[WiP] Siunitx v3

[brucemiller]

This PR begins to update the siunitx binding to version 3. It preserves the old version 2 tests as t/complex/si-v2, and updates the si case to avoid errors.

The si testcase does not yet provide a complete test of version 3 commands. Nor is the binding likely to contain all the new version 3 commands.

Once this is done, it should
Fixes #1734
and maybe even #2568, #2371.