Set esbuild target to es2021 to support Safari < 16.4 (#1737)

Author: larabr

package.json

@@ -32,7 +32,7 @@
  },
  "main": "./index.js",
  "scripts": {
- "build": "esbuild --bundle --format=esm --keep-names --outfile=index.js lib/chai.js",
+ "build": "esbuild --bundle --format=esm --target=es2021 --keep-names --outfile=index.js lib/chai.js",
  "prebuild": "npm run clean",
  "format": "prettier --write lib",
  "pretest": "npm run lint",