make djot divs generate arbitrary elements rather than just divs

this makes it possible to use elements like figure/figcaption

content/programming/tairu.tree

@@ -236,7 +236,8 @@ tags = ["all", "programming", "graphics", "javascript"]
       id = "01HQ162WWAS502000K8QZWVBDW"
     - we can split this tileset up into 16 individual tiles, each one 8 × 8 pixels; people choose various resolutions, I chose a fairly low one to hide my lack of artistic skill.
 
-    ::: horizontal-tile-strip
+    {.horizontal-tile-strip}
+    :::
     []{.metal .x-0 .y-0}
     []{.metal .x-1 .y-0}
     []{.metal .x-2 .y-0}
@@ -260,7 +261,8 @@ tags = ["all", "programming", "graphics", "javascript"]
     - the keen eyed among you have probably noticed that this is very similar to the case we had before with drawing procedural borders -
     except that instead of determining which borders to draw based on a tile's neighbors, this time we'll determine which _whole tile_ to draw based on its neighbors!
 
-    ::: horizontal-tile-strip
+    {.horizontal-tile-strip}
+    :::
     [[E]{.east} [S]{.south}]{.metal .x-0 .y-0}
     [[E]{.east} [S]{.south} [W]{.west}]{.metal .x-1 .y-0}
     [[S]{.south} [W]{.west}]{.metal .x-2 .y-0}
@@ -307,7 +309,8 @@ tags = ["all", "programming", "graphics", "javascript"]
           id = "01HQ162WWABANND0WGT933TBMV"
         - that means we'll need to arrange our tiles like so, where the leftmost tile is at index 0 (`0b0000`) and the rightmost tile is at index 15 (`0b1111`):
 
-        ::: horizontal-tile-strip
+        {.horizontal-tile-strip}
+        :::
         []{.metal .x-3 .y-3}
         [[E]{.east}]{.metal .x-0 .y-3}
         [[S]{.south}]{.metal .x-3 .y-0}