Refactor, type fixes, small rewrites

[natedunn]

I am running out the door (on vacation at the moment), but PRing this for reference, regardless for what @Nicolapps decides to do in https://github.com/get-convex/convex-helpers/pull/818.

Non-complete notes I wrote up while drafting this PR:

• First, thanks to @panzacoder and @ksinghal on writing much of this code. And ianmacartney for the original code for which we all probably referenced.
• Moved code into separate files per their functions, as single file was getting a bit unwieldy
• Fixed a number of type issues in converting convex schemas to zod. Good amount of help came from the zodvex library from @panzacoder
• Fixed builders' inferred types
• Re-added old JS doc comments for IDE reference
• Added more tests
• Changed most Zod exports to use the subpath (zod/v4/core) and adjusted subclasses to support this change. However I think there are still cases in which we need to export base zod to use certain methods (like .unwrap) and schema creation (i.e. convexToZod).
• (Needs work) Optional Ids are not correctly converting from Convex to Zod
• (Needs work) Certainly other things I haven't gotten around to testing.