fix: Missing Delete key handlers

packages/core/src/api/blockManipulation/commands/mergeBlocks/mergeBlocks.ts

@@ -19,8 +19,11 @@ export const getParentBlockInfo = (doc: Node, beforePos: number) => {
 
   // get start pos of parent
   const parentBeforePos = $pos.posAtIndex(
-    $pos.index($pos.depth - 1),
-    $pos.depth - 1,
+    // TODO: This works for blockContainer nodes as we need to traverse 2
+    // nesting levels due to the blockGroup in between. However, that's not the
+    // case for columns.
+    $pos.index($pos.depth - 2),
+    $pos.depth - 2,
   );
 
   const parentBlockInfo = getBlockInfoFromResolvedPos(
@@ -50,6 +53,27 @@ export const getPrevBlockInfo = (doc: Node, beforePos: number) => {
   return prevBlockInfo;
 };
 
+/**
+ * Returns the block info from the sibling block after (below) the given block,
+ * or undefined if the given block is the last sibling.
+ */
+export const getNextBlockInfo = (doc: Node, beforePos: number) => {
+  const $pos = doc.resolve(beforePos);
+
+  const indexInParent = $pos.index();
+
+  if (indexInParent === $pos.node().childCount - 1) {
+    return undefined;
+  }
+
+  const nextBlockBeforePos = $pos.posAtIndex(indexInParent + 1);
+
+  const nextBlockInfo = getBlockInfoFromResolvedPos(
+    doc.resolve(nextBlockBeforePos),
+  );
+  return nextBlockInfo;
+};
+
 /**
  * If a block has children like this:
  * A