chore: import code snippets for l1-l2 txn tutorial (#122)

- moves code snippets for l1-l2 txn tutorial to the `code` folder
- adds steps to run the tutorial locally

Author: sarahschwartz

code/cross-chain-tx/.env.example

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+WALLET_PRIVATE_KEY=

code/cross-chain-tx/.gitignore

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+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+lerna-debug.log*
+.vscode
+
+# hardhat artifacts
+artifacts
+cache
+
+# zksync artifacts
+artifacts-zk
+cache-zk
+
+# Diagnostic reports (https://nodejs.org/api/report.html)
+report.[0-9]*.[0-9]*.[0-9]*.[0-9]*.json
+
+# Runtime data
+pids
+*.pid
+*.seed
+*.pid.lock
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+*.lcov
+
+# nyc test coverage
+.nyc_output
+
+# Grunt intermediate storage (https://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# Bower dependency directory (https://bower.io/)
+bower_components
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (https://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directories
+node_modules/
+jspm_packages/
+
+# TypeScript v1 declaration files
+typings/
+
+# TypeScript cache
+*.tsbuildinfo
+
+# Optional npm cache directory
+.npm
+
+# Optional eslint cache
+.eslintcache
+
+# Microbundle cache
+.rpt2_cache/
+.rts2_cache_cjs/
+.rts2_cache_es/
+.rts2_cache_umd/
+
+# Optional REPL history
+.node_repl_history
+
+# Output of 'npm pack'
+*.tgz
+
+# Yarn Integrity file
+.yarn-integrity
+
+# dotenv environment variables file
+.env
+.env.test
+
+# parcel-bundler cache (https://parceljs.org/)
+.cache
+
+# Next.js build output
+.next
+
+# Nuxt.js build / generate output
+.nuxt
+dist
+
+# Gatsby files
+.cache/
+# Comment in the public line in if your project uses Gatsby and *not* Next.js
+# https://nextjs.org/blog/next-9-1#public-directory-support
+# public
+
+# vuepress build output
+.vuepress/dist
+
+# Serverless directories
+.serverless/
+
+# FuseBox cache
+.fusebox/
+
+# DynamoDB Local files
+.dynamodb/
+
+# TernJS port file
+.tern-port
+
+# Deployment files
+deployments-zk
+
+era_test_node.log
+anvil_zksync.log

code/cross-chain-tx/.npmrc

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+legacy-peer-deps=true

code/cross-chain-tx/bun.lockb

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+//SPDX-License-Identifier: Unlicense
+pragma solidity ^0.8.0;
+
+contract Greeter {
+    string private greeting;
+
+    constructor(string memory _greeting) {
+        greeting = _greeting;
+    }
+
+    function greet() public view returns (string memory) {
+        return greeting;
+    }
+
+    function setGreeting(string memory _greeting) public {
+        greeting = _greeting;
+    }
+}

code/cross-chain-tx/contracts/Greeter.sol

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+import type { HardhatUserConfig } from 'hardhat/config';
+
+import '@matterlabs/hardhat-zksync';
+
+import dotenv from 'dotenv';
+dotenv.config();
+
+const config: HardhatUserConfig = {
+  defaultNetwork: 'ZKsyncEraSepolia',
+  networks: {
+    ZKsyncEraSepolia: {
+      url: 'https://sepolia.era.zksync.dev',
+      ethNetwork: 'sepolia',
+      zksync: true,
+      verifyURL: 'https://explorer.sepolia.era.zksync.dev/contract_verification',
+      accounts: process.env.WALLET_PRIVATE_KEY ? [process.env.WALLET_PRIVATE_KEY] : [],
+    },
+    ZKsyncEraMainnet: {
+      url: 'https://mainnet.era.zksync.io',
+      ethNetwork: 'mainnet',
+      zksync: true,
+      verifyURL: 'https://zksync2-mainnet-explorer.zksync.io/contract_verification',
+      accounts: process.env.WALLET_PRIVATE_KEY ? [process.env.WALLET_PRIVATE_KEY] : [],
+    },
+    dockerizedNode: {
+      url: 'http://localhost:3050',
+      ethNetwork: 'http://localhost:8545',
+      zksync: true,
+      accounts: process.env.WALLET_PRIVATE_KEY ? [process.env.WALLET_PRIVATE_KEY] : [],
+    },
+    anvilZKsync: {
+      url: 'http://127.0.0.1:8011',
+      ethNetwork: 'localhost', // anvil doesn't support eth node; removing this line will cause an error
+      zksync: true,
+      accounts: process.env.WALLET_PRIVATE_KEY ? [process.env.WALLET_PRIVATE_KEY] : [],
+    },
+    hardhat: {
+      zksync: true,
+    },
+  },
+  zksolc: {
+    version: 'latest',
+    settings: {
+      // find all available options in the official documentation
+      // https://docs.zksync.io/build/tooling/hardhat/hardhat-zksync-solc#configuration
+    },
+  },
+  solidity: {
+    version: '0.8.24',
+  },
+};
+
+export default config;