Upgrade to latest Node LTS v18

[Jinksi]

Fixes #230

Description

How to test this PR

1. Check out this branch.
2. Install the latest LTS version of node by running nvm install from this repo's directory.
3. Run nvm use. You should now be using Node v18.x.
4. Run npm install. You should not see an Unsupported engine warning.
5. Run npm run build. The build should complete successfully.

• No npm install warning related to unsupported engines.
  • Note: 2 unsupported engine warnings occur: @es-joy/[email protected] [email protected].
    These occur due to @woocommerce/eslint-plugin and @wordpress/scripts.
• npm run build works as expected without errors.
  • Build artefacts in ./build have minor changes, presumably due to updating @wordpress/scripts.
• package-lock.json reflects any changes related to the node/npm version upgrade.

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref

[Jinksi]

Currently, when running on node v18, npm run build results in the error below.

It looks like we'll be forced to upgrade some packages starting with @wordpress/scripts.

wp-scripts build assets/src/js/index.js

node_modules/loader-runner/lib/LoaderRunner.js:114
                        throw e;
                        ^

Error: error:0308010C:digital envelope routines::unsupported