refactor: use .ts sources and vitest

[fvsch]

I wrote this library using .js files and TypeScript types in JSDoc comments, to see how far I could go that way.

It works pretty well and lets you run code with Node directly, as well as using Node's built-in test runner.

There are a few downsides though:

• Package size: this pattern ships types (in JSDoc comments) in the final package, even though those are not used at runtime.
• DX: it's still more painful to write TS in JSDoc than in .ts files.
• DX: Node's built-in test runner is okay, but a bit limited.

So now that I’ve shown myself that it’s doable and not too painful without a build step and transpilation shenanigans involved, as a matter of personal preference I’m switching to .ts sources and Vitest.

One small win: using Vitest unlocks running tests on Node 18, since we no longer depend on node:test's suite function (not available in Node 18).