Provide a flag to auto-prefix local css classes when minified

[chowey]

This addresses #3484 by providing an option to prefix css classes when minifying css modules.

I created the --local-css-prefix command line flag, e.g. --local-css-prefix=myapp-. I also exported this through the Javascript API.

You can now do e.g.

await esbuild.build({
  entryPoint: "index.js",
  outdir: "./dist",
  bundle: true,
  format: "esm",
  minify: true,
  localCSSPrefix: "myapp-", // new option
});

---

With the following input files:

index.js

import classes from "./classes.module.css";

document.getElementById("main").className = classes.localClassWithRedText;

classes.module.css

.localClassWithRedText {
  color: red;
}

This produces the following output files:

dist/index.css

.myapp-o{color:red}

dist/index.js

var e={localClassWithRedText:"myapp-o"};document.getElementById("main").className=e.localClassWithRedText;