www/yarn: Update to version 0.24.6

Other changes:

• Patch package.json to show installation was via package [1]
• Set executable bit for JavaScript scripts requiring it [1]
• Do not install Windows binaries
• Use fully qualified paths to create links in ${PREFIX}/bin/ [2]
• Create link for yarnpkg in ${PREFIX}/bin [1]

Upstream releases: https://github.com/yarnpkg/yarn/releases

Reviewed by: jlduran@gmail.com [1], mat [2]
Approved by: luca.pizzamiglio@gmail.com (maintainer),

swills (mentor, implicit)

Differential Revision: https://reviews.freebsd.org/D10696