Specify Yarn as the package manager in package.json

[TixieSalander]

Proposed changes

It have been automatically added by Yarn (runed in v1.22.22) when I've started the watch-assets task so, I think it should be added to the code base.

cf. https://yarnpkg.com/configuration/manifest#packageManager

Contributor checklist

• [x] My PR is related to \ (no issue)
• [x] I have read and understood the CONTRIBUTING guide
• [ ] I have inserted my change and a link to this PR in the CHANGELOG
• [x] I have updated documentation in ./docs/docs if a public feature has a behaviour change no behaviour change

[tchoutri]

Yeah I think it must be VSCode doing it, because I remember it happened last time we paired. Let's comply then. :)

[TixieSalander]

@tchoutri I don't think it's VSCode, I don't have it opened, remove the change, re-launch yarn and it have been added again

[tchoutri]

@TixieSalander Interesting! Which version of yarn do you use, by any chance?

[TixieSalander]

@tchoutri v1.22.22, runned inside Docker