Ensure regionalEndpoint variable is declared for s3UsEast1RegionalEndpoint

If the file is executed in node strict mode https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode#assigning_to_undeclared_variables the line fails. Typically that should not happen because the file does not declare a "use strict" at the top but with some bundlers it can still happen.

Steps to reproduce:

package.json

{
  "name": "s3sdktest",
  "version": "1.0.0",
  "dependencies": {
    "aws-sdk": "^2.1501.0"
  },
  "devDependencies": {
    "esbuild": "^0.19.7"
  }
}

test.js

"use strict";
const AWS = require("aws-sdk");
const s3 = new AWS.S3({ region: "us-east-1",  s3UsEast1RegionalEndpoint: "regional" });
s3.listBuckets().promise();

Command to bundle:

$ npx esbuild --bundle test.js --outdir=dist --platform=node --target=node18

Resulting file includes a "use strict"; in the first line from the esbuild bundler:

$ head dist/test.js 
"use strict";
var __getOwnPropNames = Object.getOwnPropertyNames;
var __commonJS = (cb, mod) => function __require() {
  return mod || (0, cb[__getOwnPropNames(cb)[0]])((mod = { exports: {} }).exports, mod), mod.exports;
};

// node_modules/aws-sdk/lib/json/builder.js
var require_builder = __commonJS({
  "node_modules/aws-sdk/lib/json/builder.js"(exports, module2) {
    var util = require_util();

That means all bundled files have strict mode enabled and not only the test.js file which declared it. It feels like this is a bug in esbuild but still we should solve it in the sdk as well.

Fixes https://github.com/aws/aws-sdk-js/issues/4486

Checklist

[ ] npm run test passes - not needed
[ ] .d.ts file is updated - not needed
[ ] changelog is added, npm run add-change - not needed?
[ ] run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed - not needed
[ ] run npm run integration if integration test is changed - not needed
[ ] non-code related change (markdown/git settings etc) - not really

aws / aws-sdk-js

Ensure regionalEndpoint variable is declared for s3UsEast1RegionalEndpoint #4531

Checklist