fix(sveltekit): rename adapter.ts -> adapters.ts in @auth/sveltekit (#12503)

rename adapter.ts -> adapters.ts in framework-sveltekit

The package.json exports ./adapters, consistent with other
packages. This meant it was not accessible for consumption
when the source file and exported files didn't match.

Co-authored-by: Thang Vu <hi@thvu.dev>

Author: emme1444