Support TOML build in VS Code

[jvican]

I think it'd be lovely if we could support code completions and diagnostics in VS Code. These are features that are typically supported out-of-the-box for JSON configuration files, but I also think we should go a little bit further than that and support coursier dependency completions: https://twitter.com/alxarchambault/status/1123878375884578816 This feature alone would make the use of seed for beginners a very enjoyable experience.

I think there's a case to consider switching from TOML to JSON if we want to ship editor support sooner and have a more lightweight implementation. VS Code supports JSONc (JSON with comments) and provides completions/red squigglies when a JSON schema is provided. I have no opinion here, I think JSON would be more amenable and intuitive to users (especifically those coming from build tools such as npm and the like), but TOML is also nice.