Prevent server leak

[eric-burel]

Is your feature request related to a problem? Please describe.
Currently: server folder or server.ts means server-only code => but we don't strictly enforce that so there's a risk of leak

Describe the solution you'd like
Prevent user from accidentally loading server-only code client-side

Describe alternatives you've considered

• An eslint rule that detect that you are not in a "server" file (no server folder/no server.ts) but you import a "server" code accidentally
• A webpack plugin that fails the client bundle if server code is detected