feat(klatre): expose utility to format dot time offset

Turns out this is an annoying thingy you sometimes to implement
independently from formatting an entire timestamp, so we expose it for
reuse.

Change-Id: I11de2823eb03849ea78fc79e2f546e413882930f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3163
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

lisp/klatre/package.lisp

@@ -10,7 +10,7 @@
 
    ;; String handling
    #:+dottime-format+ #:format-dottime
-   #:try-parse-integer
+   #:try-parse-integer #:format-dottime-offset
 
    ;; Function utilities
    #:partial))