feat: implement IsInhabited as a first-class type primitive #49

Description

Implement as a first-class matcher. It returns true if T has at least one inhabiting value (i.e. is not never), and false otherwise.

This is the one primitive that neither expect-type nor tsd provides, and it addresses a real gap: testing that a type utility doesn't accidentally collapse everything to never.

Why

expect-type has IsNever but no inverse. IsInhabited is useful for:

• Validating that a mapped type produces non-empty results
• Testing that a conditional type doesn't collapse to never
• Guarding against Extract<T, never> patterns that silently return never

Acceptance criteria

• IsInhabited<T> implemented in src/types/
• check<T>().isInhabited() chainable API
• toBeInhabited() Vitest matcher
• Tests: IsInhabited<string> → true, IsInhabited<never> → false
• Integration with existing Inhabited if already present

Risks

• Low. IsInhabited<T> is a straightforward boolean wrapper around [T] extends [never] ? false : true.

Related

• src/types/special.ts for existing special-type patterns