fix!: don't delete "main" in package.json

esbuild still doesn't support exports evanw/esbuild#187
it's uncertain as yet what impact this might have on other bundlers and
loaders, hence this is a BREAKING CHANGE.

src/package/index.js

@@ -182,7 +182,6 @@ class Package {
  const json = copy(this.pkgjson)
 
  delete json.type
- delete json.main
  json.browser = {}
  json.exports = {}
  const _join = (...args) => './' + join(...args)