refactor(sterni/mblog): add (sub)class for apple note messages

* Upon creation of an apple-note object we can check if certain fields
  we are interested in are present and of the right type etc.

  These currently are:

  - UUID (for links later)
  - Subject (title)
  - Time
  - Text part with supported MIME type

  These are then put into their own shortcut fields in the apple-note
  subclass which allows for easier access and forces us to make sure
  they are present.

* Split out everything note related into its own package. Using the new
  type, we can expose an interface which sort of makes sense.

Change-Id: Ic9d67518354e61a3cc8388bb0e566fce661e90d0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5072
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/mblog/cli.lisp

@@ -13,5 +13,6 @@
                               args))))
     (if help-p (format *error-output* "Usage: ~A~%" +synopsis+)
       (loop for arg in args
-            do (apple-note-html-fragment
-                (mime:mime-message (pathname arg)) *standard-output*)))))
+            do (note:apple-note-html-fragment
+                (note:make-apple-note (mime:mime-message (pathname arg)))
+                *standard-output*)))))