Adopt yarn version in ci

[bennypowers]

yarn has a version command which can sync sibling packages in a monorepo

https://yarnpkg.com/features/release-workflow

Lately, I've noticed that having more than one version of dev-server or test-runner-core in a project leads to annoying typescript errors. This would be less of an issue if versions were aligned. In the example linked, the recently released 0.6.0 version of junit-reporter depends on test-runner 0.11.0 when the current version is 0.11.1.

It seems to me that using yarn version in CI (perhaps during the changesets versions command) would be a partial fix here.