Improve Next.js tree shaking

[colinhacks]

Closes #4433

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().