Allow flat actions

[meldckn]

Currently, you to need both a top-level intent action (or "abstract action" in Michael terms, or "start symbol" in ensemble.js code terms) with an intent property, and a leaf node leading from that action. Can't just have a single, self-contained action node with intent and effects properties, that doesn't leadTo anything else. But the latter seems like a totally reasonable authoring pattern to use or assume is possible (like, "Hey let me author the simplest possible action").

Until this is fixed, authors should be warned that they can't do that (in supporting materials at least, and in tool—although that would require action validation, #79). Because any flat actions will simply never be possible even if all conditions are met, and no error will be printed on action load or when running getActions or anywhere, so you won't know why your action doesn't work.