Bump minimum Node version to 18 and use LTS for development #100

This unblocks updating the minimum Node version for all core packages to 18 and aligns this repo with the module template.

Please copy the module template in completing this:

• https://github.com/MetaMask/metamask-module-template/blob/deefbfb0d1b15fb2289b6d642d4ed0fb404cd1ea/package.json#L76
• https://github.com/MetaMask/metamask-module-template/blob/deefbfb0d1b15fb2289b6d642d4ed0fb404cd1ea/.nvmrc

You may need to also update the matrix in GitHub workflows so that we no longer test for older versions of Node.

Here is an example PR where we've done in this in the past: https://github.com/MetaMask/eth-method-registry/pull/57/files