use dependencies to generate build config

[shiftkey]

This PR introduces a canonical dependencies.json file that defines everything needed for generating a dugite-native package.

The goal here is to have a consistent source of truth for things like files and checksums to simplify two things:

• when upgrading dependencies, have one place to upgrade them (instead of multiple locations with their own quirks)
• we can derive all the artifacts we need from this source, using a repeatable script rather than manual processes

As part of investigating #140 I found myself wanting to read the YAML config files to use them as a source of truth, but this felt flaky because we have two configs and they often get out of sync because we forget to update one (potentially two if I keep pushing #122 along and use Azure Pipelines).

With this approach, we instead use dependencies.json and have a single generate-all-configs script that generates all the build configs using the dependencies metadata.

The next step to this is to have a corresponding npm run update-git-lfs or npm run update-git commands that query for the latest release and update dependencies.json to reflect the new bits. Then running generate-all-configs will update the configs with new values and everything will be ✨

TODO:

• address Appveyor config incompatibilies
• add generated header to YAML files to indicate they are machine-generated now and to refer to the source files if changes need to be made