Pin devDependency c8 to an exact version

[enochgroot]

package.json has "c8": "^12.0.0". Lockfile already records 12.0.0. Dev-only, not compiled into the addon.

Pin "12.0.0" (exact).

[enochgroot]

Implemented in #14 (branch fix/review-open-6-13).

[enochgroot]

Tracked upstream: msgpack#25703

Closing this fork issue as a duplicate of that ticket. Numbering on enochgroot/msgpack-node is a different sequence from msgpack/msgpack-node.