Stuctured command stability

Prior to this, there was an ad-hoc whitelist in `main.cc`. Now, every
command states its stability.

In a future PR, we will adjust the manual to take advantage of this new
information in the JSON.
(It will be easier to do that once we have some experimental feature
docs to link too; see #5930 and #7798.)

src/libutil/args.hh

@@ -236,6 +236,8 @@ struct Command : virtual public Args
 
     static constexpr Category catDefault = 0;
 
+    virtual std::optional<ExperimentalFeature> experimentalFeature ();
+
     virtual Category category() { return catDefault; }
 };