[2chen] Add 2chen.moe extractor (#2707)

* [2chen] Add 2chen.moe extractor

* change "==" to is

* fix for "test_unique_pattern_matches"

* fix regex pattern and group matching

* fix regex again

* [2chen] add 'reply_no' and 'hash' metadata and change 'filename_fmt'

also made an entry in supportedsites.md

* [2chen] unescape 'title'

* [2chen] partition() -> rpartition()

* [2chen] extract 'date' and 'name' metadata

* [2chen] remove 'offset' argument

* [2chen] do some changes

* [2chen] do some more changes

* [2chen] unescape 'name' and 'filename'

docs/supportedsites.md

@@ -13,6 +13,12 @@ Consider all sites to be NSFW unless otherwise known.
 </tr>
 </thead>
 <tbody valign="top">
+<tr>
+    <td>2chen</td>
+    <td>https://2chen.moe/</td>
+    <td>Boards, Threads</td>
+    <td></td>
+</tr>
 <tr>
     <td>35PHOTO</td>
     <td>https://35photo.pro/</td>