Support SFD?

Per remarks on https://github.com/google/fonts/pull/10490 we will need sfd2ir if sfd is a format we need to support on an ongoing basis. The other option is to convert source formats, but if we take that path the converted source needs to become the one used for modification. I do not want a long lasting `sfd | babelfont | fontc` pipeline, that means we don't compile the form used for modification, e.g. we don't compile the source.

Begin with the remarks in PR #10490 and trace how sfd2ir, babelfont, and fontc would fit together. Done means the project has a settled support or conversion path for SFD rather than an unresolved long-lived pipeline.

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