Output CJS module bundles for perspective-viewer packages

[LukeSheard]

Output a slim version of the perspective viewer packages which is targeted when consuming the package from a module bundler (e.g. webpack, rollup, etc.)

1. Reduce the scope creep of Add pre-compiled JavaScript artifacts for Perspective package #378
2. unminified, precompiled assets, will be quicker for Webpack to parse and compile than the UMD builds.
3. It will prevent duplication of dependencies of perspective-* packages when built together - leading to smaller bundle sizes for consumers.

[texodus]

Thanks for the PR! Discussed offline, this is good to merge plus some future additions