Add test for 'webpack.entrypoints' config

[t-kelly]

Follow-up to #740

• Moves webpack/config/core.js to webpack/config/parts/core.js
• Creates a new parts/entry.js
• Isolates logic for determining Template and Layout entrypoints into their own files, get-template-entrypoints.js and get-layout-entrypoints.js
• Adds test which checks if 'webpack.entrypoints' object is merged into the output of parts/entry.js

[lock]

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