feat: add block timestamps

Why adding block timestamps?

Block is the atomic unit for logseq instead of a page, with the
timestamps we can do a lot of things, for example:
1. show a blocks timeline related to a topic/project/habit.
2. get latest modified blocks from the page or the whole graph.
3. query dsl can have a (sort-by :last_modified_at) clause

It'll add both created_at and last_modified_at properties to
each block, not all users need this, so, it's an experiment feature,
we'll disable it by default.

Also, this PR simplify the block properties a lot, it might fix some
old issues related to properties.

src/main/frontend/format/block.cljs

@@ -134,6 +134,12 @@
    (= "List" (first block))
    (:name (first (second block)))))
 
+(defn- ->schema-properties
+  [properties]
+  (-> properties
+      (update "created_at" util/safe-parse-int)
+      (update "last_modified_at" util/safe-parse-int)))
+
 (defn extract-properties
   [[_ properties] start-pos end-pos]
   (let [properties (->> (into {} properties)
@@ -146,7 +152,7 @@
                                                     (util/safe-parse-int v')
                                                     v')]
                                            [k' v']))))]
-    {:properties properties
+    {:properties (->schema-properties properties)
      :start-pos start-pos
      :end-pos end-pos}))
 
@@ -379,7 +385,7 @@
                                    block
                                    {:block/meta meta
                                     :block/marker (get block :block/marker "nil")
-                                    :block/properties (get block :block/properties [])
+                                    :block/properties (get block :block/properties {})
                                     :block/file file
                                     :block/format format
                                     :block/page page