Yarn engine requirement

[javan]

Adds yarn requirement to package.jsons engines:

webpacker/package.json

Lines 9 to 11 in 09247aa

	"engines": {
	"node": ">=6.0.0",
	"yarn": ">=0.25.2"

Updates webpacker:check_yarn and webpacker:check_node to read requirements from package.json so they're not duplicated (and easily forgotten):

webpacker/lib/tasks/webpacker/check_yarn.rake

Lines 8 to 14 in 09247aa

	pkg_path = Pathname.new("#{__dir__}/../../../package.json").realpath
	yarn_requirement = JSON.parse(pkg_path.read)["engines"]["yarn"]
	requirement = Gem::Requirement.new(yarn_requirement)
	version = Gem::Version.new(yarn_version)
	unless requirement.satisfied_by?(version)

[gauravtiwari]

Shall we extract this part inside version module? And then perhaps use that to check inside tasks?

if Webpacker::Version.required_node_version?(version)
if Webpacker::Version.required_yarn_version?(version)

[javan]

I'd rather not add a new API that's only used by rake tasks, but 👍 if there's ever a broader need.

[gauravtiwari]

Cool, makes sense 👍