feat: add ---@toc (Table of Contents) generation - #25

================================================================================
Table of Contents                                           *my-plugin.contents*

First Module······················································|first.module|
Second Module····················································|second.module|
Third Module······················································|third.module|

================================================================================
First Module                                                      *first.module*

================================================================================
Second Module                                                    *second.module*

================================================================================
Third Module                                                      *third.module*