assert: make `actual` and `expected` getters

The `actual` and `expected` properties on an instance of
`AssertionError` is now a getter to prevent inspecting these when
inspecting the error. These values will be visible in the error
message and showing them otherwise would decrease the readability
of the error.

PR-URL: https://github.com/nodejs/node/pull/25250


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>