[lynxchan] add generic extractors for lynxchan imageboards (#3394)

* [lynxchan] add generic extractors for lynxchan imageboards

includes kohlchan.net, endchan.org:wq

* [lynxchan] set pop default to empty tuple

* Apply suggestions from code review

Co-authored-by: Mike Fährmann <mike_faehrmann@web.de>

docs/supportedsites.md

@@ -433,12 +433,6 @@ Consider all sites to be NSFW unless otherwise known.
     <td>Soundtracks</td>
     <td></td>
 </tr>
-<tr>
-    <td>Kohlchan</td>
-    <td>https://kohlchan.net/</td>
-    <td>Boards, Threads</td>
-    <td></td>
-</tr>
 <tr>
     <td>Komikcast</td>
     <td>https://komikcast.site/</td>
@@ -1046,6 +1040,22 @@ Consider all sites to be NSFW unless otherwise known.
     <td></td>
 </tr>
 
+<tr>
+    <td colspan="4"><strong>LynxChan Imageboards</strong></td>
+</tr>
+<tr>
+    <td>Kohlchan</td>
+    <td>https://kohlchan.net/</td>
+    <td>Boards, Threads</td>
+    <td></td>
+</tr>
+<tr>
+    <td>Endchan</td>
+    <td>https://endchan.org/</td>
+    <td>Boards, Threads</td>
+    <td></td>
+</tr>
+
 <tr>
     <td colspan="4"><strong>Nijie Instances</strong></td>
 </tr>