Add ISUPPORT HOSTLEN

It's in modern, but it's missing here:
https://modern.ircdocs.horse/#hostlen-parameter

Author: emersion

_data/isupport.yaml

@@ -335,6 +335,16 @@ values:
 
         obsolete: true
 
+    -
+        name: HOSTLEN
+        format: "HOSTLEN=[number]"
+        comment: >
+            Indicates the maximum length of an hostname in octets.
+
+        examples:
+            - "HOSTLEN=63"
+            - "HOSTLEN=100"
+
     -
         name: INVEX
         format: "INVEX[=letter]"