inspector: add promisified inspector module

The inspector module has only one method which receives a callback, but it seems like a good candidate for promisification since we even promisify it in our tests. I still have to update some tests in tests/sequential/, but I'm opening early to gather feedback while I work on those.

The goal here is to follow the same approach used in fs and dns (require('inspect').promises, lazy loading, experimental warning, etc.).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines