Improve handling shared npm dependencies between packages

[grega]

Currently both packages/blockly and packages/docs declare their own npm dependencies in their respectivepackage.json, but these can contain duplicate and / or conflicting resolutions, eg. #9671 (comment)

Any shared dependency ought to be explicitly handled in a way that prevents duplication, eg. added to a root-level package.json only.

[MannXo]

I'm interested in picking this up. Happy to discuss approach if needed.