feat: use a global symbol for `util.promisify.custom`

[ExE-Boss]

Define util.promisify.custom as:

Symbol.for("nodejs.util.inspect.custom")

rather than as:

Symbol("util.inspect.custom")

This allows custom promisify wrappers to easily/safely be defined in non‑Node.js environments.

See also:

• https://github.com/nodejs/node/issues/31647
• https://github.com/nodejs/node/pull/31672

[ljharb]

This shouldn’t land until the node one has landed.

[ExE-Boss]

@goto-bus-stop This is now shipping in Node since v12.16.2/v13.12.0.

Refs: https://nodejs.org/api/util.html#util_util_promisify_custom