👉 This PR can be tested via a pre-release version of Superstruct: npm i superstruct@1.0.5-0.

The goal of this PR is to fix Superstruct's compatibility with TypeScript's NodeNext module / module resolution settings. The simplest fix I could think of (and which was also proposed in #1211 by @ciscoheat) is to add file extensions to imports. This is currently in the PR, however, this presents several issues:

1. It breaks tests. Test are currently written in Mocha, which is really starting to show its age — it is quite hard to figure out how to make things play nice with the rest of the tooling. @ciscoheat proposed a switch to Vitest which I am 100% in favour of, but I think we need to make the switch a bit more thoughtfully: a) Make sure the tests are actually written in a more standard Vitest way and that all test are passing.
2. We have to actually use the file extensions, which — in my experience — is rather confusing to new contributors. The extension has to be .js not .ts (since TS doesn't rewrite paths), and we lose directory import. Bottom line, this feels like a non-ideal way to do this and other libs manage in a more elegant way, so we investigate how this can be done.
3. There are many environments where Superstruct can run, and any changes to module resolution should be either thoroughly tested (at least manually), or marked as a breaking change to make sure we don't break suddenly people's builds. I would really hate to drop the whole TS/Node/CommonJS/ESM mess on unsuspecting users.