feat: load config from package.json

[cpreston321]

Issue: #29

The only questions I have is

1. Do we want to just normalize it to just use the name to drive how to load the package.json or do we want to support array of keys in addition of the name. The only reason I asked is unbuild supports these keys unbuild and build. Let me know what you think.

2. If we extend from multiple directories how do we want the configFile naming convention to be ?

Thanks 💚

[pi0]

Do we want to just normalize it to just use the name to drive how to load the package.json or do we want to support array of keys in addition of the name. The only reason I asked is unbuild supports these keys unbuild and build. Let me know what you think.

Good point about unbuild! We can default to name probably and either support (a more generic aliases) option or accept string|string[] for packageJson option.