chore: use real private class fields (Node.js v12 was dropped)

[telleroutlook]

The package.json engines field now requires ^16.10.0 || ^18.12.0 || >=20.0.0, meaning Node.js 12 support was dropped long ago.

This converts the pseudo-private fields (_foo) to real ES2022 private fields (#foo) in both LightRunner and MainThreadTinypool, fulfilling the TODO comment left in the original code:

// TODO: Use real private fields when we drop support for Node.js v12

Changes:

• LightRunner._globalConfig → #globalConfig
• LightRunner._testRunners → #testRunners
• LightRunner._runtime → #runtime
• LightRunner._runTest() → #runTest()
• MainThreadTinypool._moduleP → #moduleP
• MainThreadTinypool._worker → #worker
• MainThreadTinypool._workerData → #workerData

Real private fields provide stronger encapsulation guarantees: they are enforced by the JS engine and are not accessible outside the class, unlike the _ naming convention which is purely a convention.