Add prebuilds for Node.js 16 (current LTS version)

[DavidVujic]

Add Windows and Mac OS X prebuilds for Node.js 16.

Co-authored by and originated from this Pull Request by @miroslavpokorny : #284

Description

New prebuilds for Node.js 16.

Drop prebuilds for Node.js 12.
Add CI test builds for Node.js 16, and remove for Node.js 12.

Motivation and Context

The current LTS of Node.js is v16 and supporting the latest two LTS versions should be good enough.

How Has This Been Tested?

Travis CI builds passed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).

[DavidVujic]

In the Travis CI logs for Mac OS X and Node.js 16: looks like it doesn't pick up the prebuild. 🤔

Running locally on an iMac works as expected. Is the machine in the CI using an M1 processor?

[DavidVujic]

Update: the Mac OS X prebuilds are for x64 and that is why Travis CI will run a full build. I will create a separate PR for supporting arm64 (M1 processor).