Convert private class members from `private member` to `#member`

[d13]

The private keyword is fine for TypeScript during development, but doesn't enforce private access at runtime where prefixing a class member with # will add the runtime enforcement.

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_properties

[eamodio]

I don't think we should do this, as there is a perf penalty for private members, unless there are specific cases that we NEED to keep private (certainly anything we expose outside ourselves).