With Gulp 4, we can now easily split up the gulpfile into separate task
files. Per the Getting Started documentation:

>Node's module resolution allows you to replace your gulpfile.js file
>with a directory named gulpfile.js that contains an index.js file which
>is treated as a gulpfile.js.
>@link https://gulpjs.com/docs/en/getting-started/javascript-and-gulpfiles

This commit changes the `gulpfile.js` from a file to directory and adds
an `index.js` file, which Node resolves the entry point into the directory.

This new design allows a developer to clone the UpGulp repo and name it
something like `dev`. Why? It allows all development files, tools, and
packages (such as Sass, composer, package.json, tests, node_modules,
rc configurations, etc.) to be held in one directory.