Make Lua's "exit" function a little prettier

"exit" now has a custom __tostring method, which prints an explanation
message. This is very similar to how Python achives the same
functionality:

    lua> exit
    Call exit() to exit
    lua> exit()
    > Actually leaves the REPL

src/main/resources/assets/computercraft/lua/rom/programs/lua.lua

@@ -9,9 +9,10 @@ end
 local bRunning = true
 local tCommandHistory = {}
 local tEnv = {
-    ["exit"] = function()
-        bRunning = false
-    end,
+    ["exit"] = setmetatable({}, {
+        __tostring = function() return "Call exit() to exit" end,
+        __call = function() bRunning = false end,
+    }),
     ["_echo"] = function( ... )
         return ...
     end,