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/packages.lisp

@@ -5,18 +5,33 @@
   (:documentation
    "Very incomplete package for dealing with maildir(5)."))
 
-(defpackage :mblog
+(defpackage :note
   (:use
    :common-lisp
-   :mime4cl
    :closure-html
    :who
-   :uiop)
-  (:shadow :with-html-output) ; conflict between closure-html and who
+   :cl-date-time-parser
+   :mime4cl
+   :who)
   (:import-from
    :alexandria
    :when-let*
    :when-let
    :starts-with-subseq
    :ends-with-subseq)
+  (:shadow :with-html-output) ; conflict between closure-html and who
+  (:export
+   :apple-note
+   :apple-note-uuid
+   :apple-note-subject
+   :apple-note-time
+   :apple-note-text-part
+   :make-apple-note
+   :apple-note-html-fragment))
+
+(defpackage :mblog
+  (:use
+   :common-lisp
+   :uiop
+   :note)
   (:export :main))