Add entries for extension name schemas

[kersten1]

Extension name schemas: e.g. Z* for unpriv extensions, S* for priv extensions, Za* for atomics, Zb* for bitmanip stuff, Zv* for vector extensions, Zk* for crypto extensions, Sv* for Supervisor virtual memory extensions, Sh* for supervisor items, Sm* for Machine Mode extensions, and so on. I’m no expert on the schemas, so I would suggest working with Andrew Waterman to review the above.