help codesandbox auto-detect appropriate template

Currently, we're recursively gathering the active file's content and dependencies to build our `{ files, dependencies }` structure. This works well for plain ES6 JavaScript, but not so much for more involved setups, like Svelte or Vue.

Codesandbox attempts to auto-detect an appropriate template by looking for `{ dependencies, devDependencies }` in the `/package.json` entry in the `files` object.

We should search upwards from the selected file up to and including `atom.project.getDirectories()` for a `package.json`