chore: add tasks headers

2021/solutions.livemd

···
 {:module, Day10, <<70, 79, 82, 49, 0, 0, 7, ...>>, {:parse, 2}}
 ```
 
+### Task 1
+
 ```elixir
 points = %{
   ?) => 3,
···
 ```output
 288291
 ```
+
+### Task 2
 
 ```elixir
 points = %{
···
 
   @diffs for dx <- -1..1, dy <- -1..1, dx != 0 or dy != 0, do: {dx, dy}
 
-  IO.inspect(@diffs)
-
   def lightup(map, n) do
     map
     |> Enum.reduce({map, 0}, fn
···
 ```
 
 ```output
-[{-1, -1}, {-1, 0}, {-1, 1}, {0, -1}, {0, 1}, {1, -1}, {1, 0}, {1, 1}]
+{:module, Day11, <<70, 79, 82, 49, 0, 0, 11, ...>>, {:lightup, 2}}
 ```
 
-```output
-{:module, Day11, <<70, 79, 82, 49, 0, 0, 11, ...>>, {:lightup, 2}}
-```
+### Task 1
 
 ```elixir
 draw = fn map ->
···
 ```output
 1688
 ```
+
+### Task 2
 
 ```elixir
 Stream.unfold(1, &{&1, &1 + 1})
···
 {:module, Day12, <<70, 79, 82, 49, 0, 0, 15, ...>>, {:small?, 1}}
 ```
 
+### Task 1
+
 ```elixir
 Day12.dfs(graph, "start", "end")
 ```
···
 ```output
 4167
 ```
+
+### Task 2
 
 ```elixir
 Day12.dfs2(graph, "start", "end")
···
 {:module, Day13, <<70, 79, 82, 49, 0, 0, 13, ...>>, {:draw, 1}}
 ```
 
+### Task 1
+
 ```elixir
 Day13.fold(hd(folds), input) |> MapSet.size()
 ```
···
 ```output
 802
 ```
+
+### Task 2
 
 ```elixir
 Enum.reduce(folds, input, &Day13.fold/2)
···
 {:module, Day14, <<70, 79, 82, 49, 0, 0, 13, ...>>, {:frequencies, 1}}
 ```
 
+### Task 1
+
 ```elixir
 {{_, min}, {_, max}} =
   1..10
···
 ```output
 2768
 ```
+
+### Task 2
 
 ```elixir
 {{_, min}, {_, max}} =
···
 {99, 99}
 ```
 
+### Task 1
+
 ```elixir
 shortest_paths =
   for y <- height..0//-1,
···
 ```output
 429
 ```
+
+### Task 2
 
 ```elixir
 defmodule Day15.Task2 do
···
 }
 ```
 
+### Task 1
+
 ```elixir
 defmodule Day16.Task1 do
   alias Day16.Packet
···
 ```output
 949
 ```
+
+### Task 2
 
 ```elixir
 defmodule Day16.Task2 do