jj: Add alias to insert change before or after current

Internally `jnb` runs `jj new -B@` so that it simply always insert a change
directly before the current working copy.

Likewise with `jna` for inserting after current. (Which is still
somewhat helpful since by default `jj new` will create a branch in the
tree if the new change is not the leaf node)

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

@@ -10,6 +10,8 @@ if exist lazyjj; then
 fi
 
 alias jn="jj new"
+alias jna="jj new -A@"
+alias jnb="jj new -B@"
 alias jds="jj describe"
 alias jc="jj commit"