fix: make Namespace._fns private

[ammut]

The kind of change this PR does introduce

• [x] a bug fix
• [ ] a new feature
• [ ] an update to the documentation
• [ ] a code change that improves performance
• [ ] other

Other information (e.g. related issues)

Issue: #5179

This is a variant for PR #5195, please only merge one of these! I created two PRs for the related issue, providing both suggested solutions. This one uses the index signature trick as suggested in the issue.

I tried adding a test to prevent regression, but didn't manage to reproduce the error that caused all this inside the socket.io test suite. I assume this is because we use a stricter TypeScript configuration in our own codebase, but didn't bother verifying it.