Implement RFC 0014: Enable support for running Yarn Berry apps

[thitch97]

Context

As part of the effort to support building applications which use Yarn Berry (RFC 0014), this buildpack should be modified to produce a start command which accommodates the new CLI requirements.

Details

Detection

The buildpack should check for the existence of .yarnrc.yml and, if it is present, should require either node_modules or yarn_pkgs depending on the value of the nodeLinker field. If .yarnrc.yml is absent and yarn.lock is present, node_modules should be required.

Build

The buildpack should set a start command in the format yarn node <args> ..., instead of simply node <args>.

Acceptance Criteria

Given an application which uses Yarn Berry for package management:

• The buildpack should correctly require yarn-install
• A start command in the format yarn node <args> ... should be set