add option to export KGX TSV

[justaddcoffee]

per convo in hackathon with @hrshdhgd @caufieldjh @AgranyaGitHub

We could/should consider adding an option to export KGX TSV

This will be useful for the literature -> KG workflow

[cmungall]

This is for any schema or just ones that are biolink compatible.

the general plan is for the core linkml framework to have a generic kgx-like output for any classes that are annotated as being nodes or edges

[caufieldjh]

At minimum, this should be for any schema - so it could rely on linkml functionality for yielding nodes + edges. Adding additional maps for Biolink types, slots, etc. (or maybe these can just be provided as annotations, too) would be very convenient for compatibility but not strictly necessary for now.