Fixes #176

Context

See #176, but it's currently not possible to import better-ajv-errors with some TypeScript configurations because of the exports definition in package.json.

This PR follows the recommendations from https://www.typescriptlang.org/docs/handbook/esm-node.html#packagejson-exports-imports-and-self-referencing to fix the issue.

Changes

• Copy typings file to unique files in each built folder
  • Required by TS, this is how they explain it:

    It’s important to note that the CommonJS entrypoint and the ES module entrypoint each needs its own declaration file, even if the contents are the same between them. Every declaration file is interpreted either as a CommonJS module or as an ES module, based on its file extension and the "type" field of the package.json, and this detected module kind must match the module kind that Node will detect for the corresponding JavaScript file for type checking to be correct. Attempting to use a single .d.ts file to type both an ES module entrypoint and a CommonJS entrypoint will cause TypeScript to think only one of those entrypoints exists, causing compiler errors for users of the package.

• Update exports entries to point at the correct files
• Remove typings.d.ts from the files field, since it's now copied to the included lib folder