Specify minimum engine requirement of Node 18

[mattt]

Several issues have been opened with questions or problems related to fetch and Node.js versions.

• this.fetch is not a function #69
• (node:45154) UnhandledPromiseRejectionWarning: TypeError: this.fetch is not a function #93
• Documentation: make it clearer about the node.js version to use #100
• Headers is not defined #116

In #100, I wrote:

Replicate should work on any supported version of Node provided that there's a fetch function available, so I'm not sure updating the engines for the package would be appropriate.

That comment was based on an incorrect understanding of npm. According to the docs, engine requirements are advisory, and don't prevent a client from installing a package Unless the user has set the engine-strict config flag.

This PR sets the engines key in the package.json file to specify a requirement of Node >= 18.0.0.