Set package.json scripts.start as a default process

[joshwlewis]

In #184, we updated the buildpack api version for heroku/npm to 0.6. This had the unintended side effect of the web process no longer being flagged as a default process. In earlier buildpack API versions, overwriting a default process with an undefined default would result in a process flagged as default. Newer buildpack API versions will prefer the default setting of the overwriting process.

This PR ensures the web process affiliated with package.json's scripts.start is flagged as a default process, for both yarn and npm.

Fixes #211

W-10888625

[edmorley]

(I presume you'll add an integration test for this when you write the libcnb-tests for this repo, once the remaining conversions are performed? :-))

[joshwlewis]

I presume you'll add an integration test for this when you write the libcnb-tests for this repo

Yes, that's the plan. The npm and yarn buildpacks will be completely replaced (including tests), hopefully soon.