Spreadsheet: Setup and prepare for cell types

This commit adds a generic interface for cell types and hooks it up.
There is no way to set these from the UI, and so they're not saved
anywhere yet.
Also implicitly converts numeric values (strictly integers) to numeric
javascript values, as numbery-looking + numbery-looking === string is
not very interesting. :^)

Applications/Spreadsheet/JSIntegration.cpp

@@ -49,7 +49,7 @@ JS::Value SheetGlobalObject::get(const JS::PropertyName& name, JS::Value receive
         if (auto pos = Sheet::parse_cell_name(name.as_string()); pos.has_value()) {
             auto& cell = m_sheet.ensure(pos.value());
             cell.reference_from(m_sheet.current_evaluated_cell());
-            return cell.js_data();
+            return cell.typed_js_data();
         }
     }