Unify configuration file formats

[chrispcampbell]

Originally there was only one "config" file format used by SDE, which was the JSON "spec" file (e.g. mymodel_spec.json). This file allows you to define which input and output variables are used in the generated C code, and supports a number of other things, like mappings/paths for DAT files, etc.

When I added support for the sde dev and sde bundle commands, I introduced a new "config" file format (sde.config.js) that provides a superset of the functionality of the original JSON "spec" files. It serves a similar purpose, but allows for defining plugins and other things that require running code at build time.

Before we get to 1.0, it would be nice to resolve the differences between the two and clarify when to use one format over the other. My thinking is that a JSON format is fine for simple use cases (using sde generate) where there is no need for plugins. And the JS format is good for any use case (either sde generate or sde {dev|bundle}).

It would be nice if there was a single naming convention (for example, sde.config.js[on]) , and a smooth migration path if you want to start with JSON and work up to using JS, similar to how many other tools work in the JS ecosystem (ESLint's config file formats are one example).