Ensure that the name indicated by `elided_fallback_name_id` exists

[Hoolean]

This field can be set to any ID, and so we should assert that a name record with that ID exists to avoid creating an invalid table.

Even better: we could accept a string value in this position, and handle creating or re-using a name record automatically.

[belluzj]

Accepting a string would match the Designspace 5 and otBuilder:

https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#axes-element

https://github.com/fonttools/fonttools/blob/cdfac571ac70746f47f527328f8bc8eebc0aba9d/Lib/fontTools/otlLib/builder.py#L2786-L2788