Add marked in peerDepencies, and switch main in package.json to es build

[caub]

the problem with https://github.com/wurdcms/wurd-web/blob/master/package.json#L7 is webpack will use this bundle https://github.com/wurdcms/wurd-web/blob/master/dist/wurd.js that already packs marked, so it can duplicate marked if the app is also using marked in its dependencies

We should put marked in peerDependencies, and let webpack pick the ESM build by changing the main entry in package.json

Note: it should also be easy to avoid the get-property-vaue dependency as it's just

const get = (o, path) => path.split('.').reduce((obj, k) => obj[k], o);