🎨 Create block decorations in TextEditor

This is just an experiment, although I like that we can hide some information
(irrelevant to the user) behind a clean API.

src/text-editor.coffee

@@ -1398,6 +1398,13 @@ class TextEditor extends Model
   Section: Decorations
   ###
 
+  addBlockDecorationForScreenRow: (screenRow, element) ->
+    @decorateMarker(
+      @markScreenPosition([screenRow, 0], invalidate: "never"),
+      type: "block",
+      element: element
+    )
+
   # Essential: Add a decoration that tracks a {TextEditorMarker}. When the
   # marker moves, is invalidated, or is destroyed, the decoration will be
   # updated to reflect the marker's state.