Add JavaScript library that includes an `icons` object, `toSvg` function and `replace` function.

Remove icon categories to simplify npm package.

BREAKING CHANGE: The path to individual SVG files has changed. All SVGs now live on the same level
inside the `icons` directory. For example, `icons/core/home.svg` is now `icons/home.svg`.

This will allow each path to be individually manipulated with CSS.

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