Apply general code formatting to some pages in the tutorial section

tutorials/plugins/editor/import_plugins.rst

@@ -51,12 +51,15 @@ when needed:
     tool
     extends EditorPlugin
 
+
     var import_plugin
 
+
     func _enter_tree():
         import_plugin = preload("import_plugin.gd").new()
         add_import_plugin(import_plugin)
 
+
     func _exit_tree():
         remove_import_plugin(import_plugin)
         import_plugin = null
@@ -90,6 +93,7 @@ Let's begin to code our plugin, one method at time:
     tool
     extends EditorImportPlugin
 
+
     func get_importer_name():
         return "demos.sillymaterial"
 
@@ -178,8 +182,10 @@ good practice to use an enum so you can refer to them using names.
     tool
     extends EditorImportPlugin
 
+
     enum Presets { DEFAULT }
 
+
     ...
 
 Now that the enum is defined, let's keep looking at the methods of an import