library/lua/dfhack.lua interactive interpreter banner: list shortcuts

When the `lua` interactive interpreter is started, the help banner now
lists the shortcuts (that is, variable names with special handling)
already implemented in `library/lua/utils.lua` `df_shortcut_var()`.
These keywords allow the interpreter to use the currently-selected
object in the DF UI.  They are currently undocumented, as far as I
can tell.  (The `lua` interpreter documentation does contain two hints
in the Examples.)

Author: SilasD

library/lua/dfhack.lua

@@ -855,6 +855,8 @@ function dfhack.interpreter(prompt,hfile,env)
               " '^ foo' => 'printall_recurse(foo)'\n"..
               " '@ foo' => 'printall_ipairs(foo)'\n"..
               "All of these save the first result as '_'.")
+        print("These keywords refer to the currently-selected object in the game:")
+        print(" unit item plant building bld job workshop_job wsjob screen scr")
         print_banner = false
     end