Handle changes to stats made in webpack 5

[MLoughry]

This change primarily addresses two main changes made to the stats JSON in webpack v5:

• A module can be concatenated to more than one module when building multiple entries, so updated deriveGraph to no longer assume that each module would be unique.
• stats.namedChunkGroups[*].assets[*] was a string in webpack 4, but is now a { name: string, size: number } in v5. Updated deriveChunkGroupData and its tests to handle this change in a non-breaking way.

I also added a .prettierrc file (as the defaults for the Prettier VSO plugin were causing unwanted changes), and updated tsconfig.json and the engines field in package.json to handle more recent Object methods.