Add descriptive warning for invalid JSON in package.json

Of the builds that fail to deploy on Heroku, between 5-10% of them fail because `package.json` is not valid JSON.

This error currently looks like this:

```
invalid-package-json git/master
❯ git push heroku master
Counting objects: 72, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (33/33), done.
Writing objects: 100% (72/72), 42.49 KiB | 0 bytes/s, done.
Total 72 (delta 31), reused 64 (delta 29)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Node.js app detected
remote: parse error: Expected separator between values at line 9, column 14
remote: ! Unable to parse package.json
remote:
remote:
remote: -----> Build failed
remote: parse error: Expected separator between values at line 9, column 14
remote: parse error: Expected separator between values at line 9, column 14
remote:
remote: We're sorry this build is failing! You can troubleshoot common issues here:
remote: https://devcenter.heroku.com/articles/troubleshooting-node-deploys
remote:
remote: If you're stuck, please submit a ticket so we can help:
remote: https://help.heroku.com/
remote:
remote: Love,
remote: Heroku
remote:
remote: ! Push rejected, failed to compile Node.js app.
remote:
remote: ! Push failed
remote: Verifying deploy...
remote:
remote: ! Push rejected to sheltered-hollows-52627.
remote:
To https://git.heroku.com/sheltered-hollows-52627.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/sheltered-hollows-52627.git'
```

We can do much better than this.

Start by locating the buildpack path that parses package.json and emits the error shown in the issue. Use the sample deployment output as the baseline, and consider the work complete when invalid JSON produces a clearer warning than the current generic parse failure.

Written by the indexing model from the issue text.