feat(jj): add `su` alias for `jj squash`

Signed-off-by: Seongmin Lee <[e-mail]>

jj/config.toml

···
 "$schema" = "https://jj-vcs.github.io/jj/latest/config-schema.json"
 
+[aliases]
+su = ["squash"]
+
 [ui]
 default-command = ["log", "--no-pager"]
 should-sign-off = true