[Experimental] Use yarn.build to build and test the framework

[jtoar]

This PR uses yarn.build to build and test the framework. This is in place of lerna.

Note that this PR doesn't totally rip out lerna as we still use it for publishing.

yarn build and test are yarn 3 plugins. They run the build and test scripts defined in the package.json of each package in the workspace. They understand a package's dependency graph, and also have a built-in caching mechanism that avoids rebuilding/retesting a package if it hasn't been modified since since the last build/test (though this can be easily overruled via --ignore-cache).

-> build

yarn.build--build.mp4

-> test

yarn.build--test.mp4

Although merging this PR would mean that yarn build and yarn test always refers to the plugin, if the original build/test scripts are ever needed, they're still accessible via yarn run build and yarn run test.

Questions:

• I haven't added back build:watch yet (at the framework level; each package has it though). does yarn build have a built-in watching feature?
• how does the caching mechanism actually work?

Also see:

• Yarn 3 + Framework Tools: Potential next-step improvements #3516 (comment)

[jtoar]

Will try to revisit this at some point, but closing for now as there are other higher-priority yarn-related improvements that could be made.