jj: Add an alias to abandon a commit

I have begun abandoning more commits recently, so this seems a good time
to introduce an alias for it. It is not _so_ common as to require a 1 or
2-letter alias so we can go with the mnemonic `jab` for jj abandon.

vcs/jj/config/sh/alias.d/jj.sh

@@ -41,6 +41,8 @@ alias jsi="jj squash --interactive"
 alias ju="jj undo"
 # allows you to split the current change into multiple
 alias ji="jj split"
+# quickly get rid of a change
+alias jab="jj abandon"
 
 # revset info
 alias J="jj log -r 'all()'" # mirror default command being log