api: Implement prefix pattern

[timbray]

Note that the following are equivalent

{ "a": [ {"shellstyle": "foo*"} ] }

{"a": [ {"prefix": "foo"} ] }

It's already special-cased in makeShellStyleAutomaton. So there are a variety of ways this could be done, ranging from just re-writing the pattern to fixing up the switch on ValueType in AddPattern. But no new automaton construction should be required.