src: use libuv to get env vars

This allows us to remove OS-dependent code.

                                                           confidence improvement accuracy (*)    (**)   (***)
     process/bench-env.js operation='delete' n=1000000                    3.57 %      ±10.86% ±14.46% ±18.85%
     process/bench-env.js operation='enumerate' n=1000000        ***    -14.06 %       ±7.46%  ±9.94% ±12.96%
     process/bench-env.js operation='get' n=1000000                      -7.97 %      ±11.80% ±15.70% ±20.45%
     process/bench-env.js operation='query' n=1000000                    -1.32 %       ±8.38% ±11.17% ±14.58%
     process/bench-env.js operation='set' n=1000000                      -0.98 %       ±9.63% ±12.81% ±16.68%

The drop in enumeration performance is likely due to the large
number of extra allocations that libuv performs. However, enumerating
process.env should generally not be a hot path in most applications.

PR-URL: https://github.com/nodejs/node/pull/29188


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>