Move core-js into dev-dependency

[codysherman]

Confirm whether core-js should be a dev dependecy or not, and if so, move it into the proper location and re-run yarn.

[sammoore]

I don't think this matters, since the project is built via Webpack (by the Vue CLI) before our code can be run anyway; core-js is loaded when the project is compiled, which implies use of other devDependencies, right? That said, given the pattern followed with other dependencies, I think it makes sense in devDependencies since it's used for polyfilling (just "compile-time overhead" to this project) and not imported directly in any source (to my knowledge).