n-api: create functions directly

Avoid using `v8::FunctionTemplate::New()` when using
`v8::Function::New()` suffices. This ensures that individual functions
can be gc-ed and that functions can be created dynamically without
running out of memory.

Backport-PR-URL: https://github.com/nodejs/node/pull/22202
PR-URL: https://github.com/nodejs/node/pull/21688


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>