Configure npm workspaces so that `core` (and later, plugins) are each in a workspace

[rachel-fenichel]

This is part of the monorepo project.

For reference: npm workspaces documentation

• Decide whether to put all plugins into a plugins subdirectory or have each plugin parallel to core.
• @maribethb to add more detail based on previous work in chore: modernize workspaces setup using yarn blockly-samples#1597