benchmark: create napi benchmark directory

Move C++ benchmark useful for NAPI to its own directory. This will
isolate the benchmark so it can be excluded from testing that applies to
all other benchmarks but not this one.

PR-URL: https://github.com/nodejs/node/pull/21046


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