organize `server/utils.ts` functions so that they aren't being imported on the client

[vincanger]

We're importing requireNodeEnvVar into the payment/plans.ts and then some of these variables are being imported onto the client, causing clientside errors if the user extends the utils.ts file with a serverside only function.

See this convo for more info: https://discord.com/channels/686873244791210014/1339549500158771250/1339549500158771250