[FIX] FixedNumber private properties

[RauppRafael]

Updated FixedNumber's properties to use typescript's private keyword instead of the # notation.

This fixes issue #4131.

I've kept public functions without the public keyword so I don't make unwanted changes, but maybe we should add it for consistency when we have private variables

[ricmoo]

The private keyword and private members are very different things. :)

The private keyword is just a hint to the TypeScript compiler that it should not allow other TypeScript things to access this value; however, casting can circumvent this and for anyone using JavaScript without TypeScript they are unrestricted and do not even receive a warning what they are doing will break the world.

A private member is protected by the JavaScript runtime, to prevent access and prevent tampering with the internal state that could result in unexpected behaviour.

It seems like the library wrapping the objects mentioned in #4131 needs to be updated to properly reflect values onto the correct instance… Here are the docs on proper Proxy reflection to ensure supporting instances with private members… It should be an easy change in the target library and something three would likely want to support, since private members are used in quite a few places, not just Ethers…

[ricmoo]

Closing this, as the private fields are much safer, in general. In v7, I plan to switch to using WeakMaps and manually manage private members.