Drop rollup-plugin-ts, use updated addon blueprint

[simonihmig]

Update the addon boilerplate to align with embroider-build/addon-blueprint#136.

[ijlee2]

I was wondering if the *.d.js is a typo.

In my addons, I wrote the following for "." and "./*":

"exports": {
  ".": {
    "types": "./declarations/index.d.ts",
    "default": "./dist/index.js"
  },
  "./*": {
    "types": "./declarations/*.d.ts",
    "default": "./dist/*.js"
  },
  "./addon-main.js": "./addon-main.cjs"
},