src: move node::errno_string into node_errors.h/cc

Move `node::errno_string` into node_errors.h/cc and move it into
the `node:errors` namespace to reduce the size of the header.
It's not on any performance-critical code path so does not need
to be inlined.

PR-URL: https://github.com/nodejs/node/pull/25396


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