Modularize app build logic

[Siilwyn]

In the vein of https://github.com/superfly/fly/issues/141 I'm proposing to publish core/src/utils/build.ts as a module @fly/build. The logic to 'get' the webpack config and compile the assets is something I have mostly duplicated 1:1 in my app. Since it's quite specific and build.ts doesn't seem to have many internal dependencies it seems like a good fit and starting point.

Thoughts on this?

Two internal imports are:

• import log from "../log"
• import { v8envModulePath } from "../v8env" since v8env in turn imports isolated-vm it becomes quite big.

How to handle these, any ideas? They both are only a few lines of code, since v8envModulePath is only used in test and actually appended with a path the most pragmatic solution would be to inline it in my opinion.

[mrkurt]

This sounds like a great idea to me, I'm all for it. I think it will be pretty straightforward with our lerna setup too. If you feel like tackling it I'd be happy to merge + ship a new version.