feat(decorations): support signs

Add the following options to extmarks:
  - sign_text
  - sign_hl_group
  - number_hl_group
  - line_hl_group
  - cursorline_hl_group

Note: ranges are unsupported and decorations are only applied to
start_row

src/nvim/marktree.h

@@ -87,6 +87,11 @@ static inline bool mt_end(mtkey_t key)
   return key.flags & MT_FLAG_END;
 }
 
+static inline bool mt_start(mtkey_t key)
+{
+  return mt_paired(key) && !mt_end(key);
+}
+
 static inline bool mt_right(mtkey_t key)
 {
   return key.flags & MT_FLAG_RIGHT_GRAVITY;