Add support for Jest v29.

Jest v29 drops support for Node v12/v17, so I've updated the
package.json to contain the same Node support as Jest.

The only other major breaking change in Jest v29 is that the default
snapshot format has changed slightly, but that only changes the stored
jest snapshots slighty (see https://jestjs.io/blog/2022/08/25/jest-29).

BREAKING CHANGE: Drop support for Node v12 and Node v17,
                 as Jest v29 does not support these versions.

Node v16 is the current active LTS release of Node.JS

Co-authored-by: Jamie King <hello@jamieking.me>