Node.js 20 support

Nobody has claimed this yet.

Reproduction:

```sh
echo "import { test } from 'uvu'; test('one', () => {}); test.run()" > index.test.ts
npm install uvu tsm
npx tsm ./node_modules/.bin/uvu
```

Node.js 19.9.0 output:

```
(node:140378) ExperimentalWarning: Custom ESM Loaders is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:140378) DeprecationWarning: Obsolete loader hook(s) supplied and will be ignored: getFormat, transformSource
(node:140378) DeprecationWarning: Obsolete loader hook(s) supplied and will be ignored: getFormat, transformSource
index.test.ts
• (1 / 1)

Total: 1
Passed: 1
Skipped: 0
Duration: 0.26ms
```

Node.js 20.0.0 output:

```
(node:140181) ExperimentalWarning: Custom ESM Loaders is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:140181) ExperimentalWarning: Custom ESM Loaders is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
(node:140181) DeprecationWarning: Obsolete loader hook(s) supplied and will be ignored: getFormat, transformSource
```

The issue names no source file or dedicated test. Start by running the provided uvu/tsm reproduction on Node.js 19.9.0 and 20.0.0, then trace the loader entry points used by `npx tsm ./node_modules/.bin/uvu`; done means the one-test command behaves correctly on Node.js 20.

Written by the indexing model from the issue text.