Closes #4433

I landed some changes that improve the Next.js treeshaking story. I'm pretty astonished at how far Webpack/Next.js is lagging behind Rollup here. Lots of idiosyncrasies.

As always with tree-shaking issues the first thing you should try is a wildcard export:

import * as z from "zod"  // wide support for tree-shaking 
import { z } from "zod"      // not treeshakable by Webpack (Next.js) and esbuild

This is still necessary with both Webpack (at least Next.js) and esbuild. That's pretty disappointing.

Notes on the fixes:

• Pieces of each locale were getting loaded. Not the whole error map though, just some top-level objects used for word mapping. In fact all top-level objects in the codebase (exported or not) would always get bundled. In the case of locales this was fixed by declaring these as intermediates inside an "error function factory".
• Various constructed regexes were not marked with @__PURE__.
• The z.stringbool APIs were bound versions of the Zod Core _stringbool factory, which apparently wasn't shaking. This pulled in the implementations of $ZodUnknown, $ZodBoolean, and $ZodPipe.
• The fact that $ZodFunction class has methods that return instances of itself threw Webpack for a loop. Dodged this with new inst.constructor().