yarn build fails

[afirth]

probably just me (tsc noob), but perhaps worth documenting how to build this repo to encourage contributions I had to:

npm i @actions/exec
npm i @actions/core
npm i @actions/io
npm i @actions/github
yarn build

yarn add or yarn install was not sufficient to get the bundles to build, and without the npm calls I get a bunch of errors on

yarn build
[...]
 > lib/plugins-add/index.ts: error: Could not resolve "@actions/core" (mark it as external to exclude it from the bundle)
    1 │ import * as core from "@actions/core";
      ╵                       ~~~~~~~~~~~~~~~