Async API / Support loading config from ESM

[coreyfarrell]

The loadConfig function needs to become async. This will not be an issue for nyc since the main process will run this before anything else happens and we never load config during exit handlers. The reason for this is that we will need to support loading ESM from nyc.config.js, and that is async.

This will initially be difficult to support from babel-plugin-istanbul. For now when the babel plugin needs to load config from disk we'll need to execSync a script that loads and dumps the config. Eventually we'll be able to create an async pre in the plugin but that will depend on babel/babel#10507 and I'm sure at least one follow-up. It's likely off-nominal for our babel plugin to load config from disk, normally it's run under nyc where process.env.NYC_CONFIG is set and used by the babel plugin.