Keep package.json and yarn.lock in sync with CI

[dsingleton]

yarn check:

Verifies that versions of the package dependencies in the current project’s
package.json matches that of yarn’s lock file.

https://yarnpkg.com/lang/en/docs/cli/check/

Check this in CI, rather than adding to the test script, as it allows someone
to to basic things without yarn being a hard requirements. They can still:

• run locally and install deps with npm
• make changes and run tests using npm, as long as they don't change packages

If they add a package via npm add, or package.json then this CI check will
flag it.