feat(setup): add script to add shallow engine262 submodule

Introduces `setup/init_engine262.sh`, a Bash utility that clones the `engine262` repository as a shallow submodule (depth 1) into `./engine262`. The script adds the submodule, initializes it, and provides a usage comment. It ensures robust execution with `set -euo pipefail`.

setup/init_engine262.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Shallow clone the engine262 repository as a git submodule
+# Usage: ./add_engine_submodule.sh
+# This will add the submodule at ./engine262 with a depth of 1 (shallow clone)
+
+set -euo pipefail
+
+REPO_URL="https://github.com/bendtherules/engine262"
+TARGET_DIR="./engine262"
+
+# Add the submodule with shallow clone
+git submodule add --depth 1 "$REPO_URL" "$TARGET_DIR"
+
+# Initialize and update the submodule (fetches the shallow copy)
+git submodule update --init --depth 1 "$TARGET_DIR"
+
+echo "Submodule added and initialized at ./$TARGET_DIR"