add .cts and .mts type declerations for EACH built format

[spencercap]

recent changes in tsup versions >=7.1.0 have introduced multiple built ts type dec files per format. see this PR: egoist/tsup#934

and also this comment about how to adjust package.json's exports block accordingly:
total-typescript/shoehorn#9 (comment)

{
  "exports": {
    ".": {
      "import": {
        "types": "./dist/index.d.mts",
        "default": "./dist/index.mjs"
      },
      "require": {
        "types": "./dist/index.d.ts",
        "default": "./dist/index.js"
      }
    }
  }
}