module-format abstractions

[FND]

currently we simply pass format and moduleName through to Rollup, which isn't ideal in terms of sustainability:

    format: "cjs",
    moduleName: "MYAPP"

an explicit mapping seems better and also more readily understandable:

    format: "CommonJS", // case-insensitive
    exports: "MYAPP"

this would also mean we could do some validation, e.g. in the example above:

CommonJS bundles do not require or use the exports name

[FND]

done: #47