fix: Backspace at the beginning of code block should convert to paragraph

shared/editor/nodes/CodeBlock.ts

@@ -1,4 +1,5 @@
 import { NodeType } from "prosemirror-model";
+import backspaceToParagraph from "../commands/backspaceToParagraph";
 import { selectAll } from "../commands/selectAll";
 import CodeFence from "./CodeFence";
 
@@ -13,6 +14,7 @@ export default class CodeBlock extends CodeFence {
 
   keys({ type }: { type: NodeType }) {
     return {
+      Backspace: backspaceToParagraph(type),
       "Mod-a": selectAll(type),
     };
   }