Improved error messaging for conflicting link defs in schema.ts

[markyfyi]

The schema planner doesn't account for a sneaky invariant: what if the user has a link def, but in the opposite direction of one in the database? As in, the forward and reverse identities are swapped. Right now, we catch this, but throw a misleading "check your schema file for dupes" error. In this change, I add some code to check for this case, and throw a (hopefully) more informative error.

To repro - go to sandbox/cli-nodejs/instant.schema.ts and swap the forward and reverse props of one of the links.

[github-actions]

View Vercel preview at instant-www-js-backwards-link-error-jsv.vercel.app.

[nezaj]

LGTM w/ one nit.

I like the informative error, Stripe goes pretty with some of their error messages and I think it's a really nice user experience

[stopachka]

LGTM!