core: add public Lighthouse types entrypoint

[KirtiRamchandani]

Problem

core/index.js exports both the Audit class and the LH namespace types from types/lh.js. TypeScript consumers who need the LH.Audit namespace currently have to deep-import lighthouse/types/lh.js, which looks like an implementation path.

Fixes #16923.

Root cause

The package root has a name collision between the runtime Audit class export and the type namespace export. Renaming either export would be breaking, and the existing deep import is not an obvious public API.

Solution

Add a top-level type-only types.js entrypoint that re-exports types/lh.js, include it in the declaration build, and cover the entrypoint in the module test. Consumers can now use import type { Audit } from 'lighthouse/types.js' without relying on the nested implementation path.

Tests

• corepack yarn type-check
• corepack yarn build-report --flow --standalone
• corepack yarn mocha --testMatch core/test/index-test.js
• corepack yarn eslint core/test/index-test.js types.js --quiet
• git diff --check
• Verified generated declaration content for .tmp/tsbuildinfo/types.d.ts
• Verified a TypeScript API smoke test can import Audit and Flags from the generated root types.d.ts shape