Enable MSBuild Bundle props/targets to support multiple bundles

Summary

Right now out props/targets for defining and building the JS bundle assumes that each app only has one bundle. However, we have a case within our own native integration test projects where we generate multiple bundles via defining multiple <JsBundleEntry> items, and multiple bundles are created via a custom bundle target with multiple inputs.

Instead we should standardize our props/targets on that, even if most apps only have one bundle entry.

Motivation

Rather than have this useful logic hidden within our test code, we could share it for app projects that require more than one bundle. While the default template should only set up one entry (pointing at the standard index.windows.js entry file) it would make it a lot easier for app devs to just add new entries in their project and end up with more bundles as part of their build.

Basic Example

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Open Questions

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Start with the existing props/targets for defining and building JS bundles, then compare them with the custom bundle target used by the native integration test projects. Trace how multiple JsBundleEntry items and inputs are handled. Done means app projects can define multiple entries and produce multiple bundles, while the default template still uses one index.windows.js entry.