Switch to `isolatedDeclarations`

[colinhacks]

• Adds explicit types for all exports
• Lays the groundwork for publishing to JSR later
• Lays the groundwork for faster builds later (still using tsc for now)
• Prevents accidental .d.ts blowups from inferred recursive types

I disabled it inside zod/tests and benchmarks because there are some utilities/schemas that would have been annoying to declare types for.