Add an exports map to package.json

[dzearing]

TLDR: this package is missing an exports map in package.json and needs one.

When deep imports like dom-helpers/addClass are allowed, we would like them to be explicitly specified in the package.json exports map. This provides some safety features:

• Consumers can't import something you don't expect to be an api surface
• You can safely reorganize your package (move things around, rename things, change filename casing) without breaking consumers, since exports maps allow the producer to map how the import resolves.
• You can provide both commonjs and esm entry points without requiring the consumer to remap things in webpack config should that want to use another module format
• It becomes possible to ditch the nested package.jsons, reducing complexity of understanding how imports resolve to source files.

Example exports map:

{
  "exports": {
    ".": {
      "types": "./lib/index.d.ts",
      "require": "./lib-commonjs/index.cjs",
      "import": "./lib/index.js"
    },
    "./addClass": {
      "types": "./lib/addClass.d.ts",
      "require": "./lib-commonjs/addClass.cjs",
      "import": "./lib/addClass.js",
   },
   // etc
}