Investigate using yarn instead of npm

[slaFFik]

Lots of reports, that yarn is faster.

​https://twitter.com/netweb/status/785992895106912256:

NIce, 4x improvement for the first run and 18x for subsequent runs with @buddypressdev

Some more discussion here: ​https://bpdevel.wordpress.com/2016/10/28/buildtest-tooling/

Yoast, Jetpack are using yarn already. Calypso doesn’t use it yet, but may at some point, see ​https://github.com/Automattic/wp-calypso/pull/8660

[slaFFik]

@DJPaul, @netweb
I've decided to create a ticket to formalize this investigation that Paul showed interest in recently.

[netweb]

Related: ​#WP38603

​https://core.trac.wordpress.org/ticket/38603#comment:4:

Yarn currently is very new and very shiny...

Enough complications arouse in ​#WP38657 with 4.7's npm-shrinkwrap.json and as Yarn ​doesn't currently behave nicely with NPM's shrinkwrap process let's wait a while until it's a little less new and shiny...

For those reasons, moving to future release until Yarn gains a few fixes that will help rather than hinder us.

[netweb]

Related: #7535 Add npm package-lock.json for npm v5.x

[slaFFik]

WordPress core decided to use npm instead of yarn, because the difference in speed is not that big after the years. See ​https://core.trac.wordpress.org/ticket/38603#comment:14.

So I suggest to stick to the proven and already working solution (especially as npm install isn't run so often).