add exports type map to package.json

[noahgrant]

Expands the exports field in package.json from a bare string to a subpath map:

  "exports": {    
    ".": {                                                         
      "types": "./resourcerer.d.ts",                               
      "import": "./build/index.js"
    }
  }                                                                

This allows TypeScript-aware bundlers and Node's package resolution to discover types automatically via the exports map,
rather than relying solely on the top-level "types" field. Better compatibility with tools that honor exports conditions (e.g.
TypeScript's moduleResolution: bundler or node16).