implement draft/pre-away (#2044)

* implement draft/pre-away
* clean up some subtleties in auto-away aggregation.
* consistently apply auto-away only to always-on
* `AWAY *` should not produce user-visible changes wherever possible

gencapdefs.py

@@ -195,6 +195,12 @@ CAPDEFS = [
         url="https://github.com/ircv3/ircv3-specifications/pull/503",
         standard="proposed IRCv3",
     ),
+    CapDef(
+        identifier="Preaway",
+        name="draft/pre-away",
+        url="https://github.com/ircv3/ircv3-specifications/pull/514",
+        standard="proposed IRCv3",
+    ),
     CapDef(
         identifier="StandardReplies",
         name="standard-replies",