Bumps TypeScript to 5.9 (#6889)

## What's the problem this PR addresses?

I'm looking at migrating to TypeScript-go whenever it supports PnP; to
that end we first need to upgrade to the latest stable TypeScript
releases.

## How did you fix it?

Mostly just a matter of upgrading `@types/node` and updating the various
types in FakeFS that got changed.

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Author: arcanis