fix: serve CJS runtime with ESM type declarations

[angeloashmore]

Resolves: #129, #105

Description

Fixes ERR_MODULE_NOT_FOUND errors on Node.js 22+ and 24+ caused by ESM bare specifier imports of next/script, next/navigation, etc.

The exports map now serves CJS for runtime (avoiding bare import issues) while pointing the types condition to .d.ts (ESM) declarations, ensuring TypeScript resolves @prismicio/client and next/* types correctly.

Root cause and how this was tested

The package previously used require/import conditional exports, serving .cjs for CJS consumers and .js (ESM) for ESM consumers. The ESM output contains bare specifier imports like import Script from "next/script" which fail under strict ESM resolution in Node.js 22+/24+ because Next.js subpaths don't resolve without file extensions in ESM mode.

Prior fix attempts (#106, #125, #128) oscillated between CJS-only and dual formats. CJS-only fixed runtime but created a type resolution problem: .d.cts type declarations resolve dependencies in CJS mode (require condition), and if a dependency like @prismicio/client is ESM-only, TypeScript silently degrades those types to any (verified with tsc --traceResolution).

The fix uses types/default conditions instead of require/import:

".": {
    "types": "./dist/index.d.ts",
    "default": "./dist/index.cjs"
}

• Runtime: Both require() and import() resolve to .cjs — no bare import errors.
• Types: TypeScript uses the types condition → .d.ts → resolves in ESM mode → @prismicio/client and next/* types resolve correctly with full type safety.

This was tested by creating isolated test projects with moduleResolution: "node16" and "bundler", verifying type resolution traces, and confirming that types do not silently degrade to any.

Also set exports: false in tsdown.config.ts to prevent tsdown from overwriting the custom exports map during builds.

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.