Provide global function quarto_assert

This behaves mostly like the built-in `assert`, but respects the general
Quarto scaffolding.

src/resources/filters/ast/customnodes.lua

@@ -337,13 +337,7 @@ _quarto.ast = {
         end
         local node = node_accessor(table)
         local t = pandoc.utils.type(value)
-        -- FIXME this is broken; that can only be "Block", "Inline", etc
-        if t == "Div" or t == "Span" then
-          local custom_data, t, kind = _quarto.ast.resolve_custom_data(value)
-          if custom_data ~= nil then
-            value = custom_data
-          end
-        end
+        quarto_assert(t ~= 'Div' and t ~= 'Span', "")
         if index > #node.content then
           _quarto.ast.grow_scaffold(node, index)
         end

src/resources/filters/common/error.lua

@@ -15,8 +15,15 @@ function fail(message, level)
   end
 end
 
-function internal_error()
-  fail("This is an internal error. Please file a bug report at https://github.com/quarto-dev/quarto-cli/", 5)
+function internal_error(msg, level)
+  fail((msg and (msg .. '\n') or '') ..
+    "This is an internal error. Please file a bug report at https://github.com/quarto-dev/quarto-cli/", level or 5)
+end
+
+function quarto_assert (test, msg, level)
+  if not test then
+    internal_error(msg, level or 6)
+  end
 end
 
 function currentFile()