refactor: convert local images to data URIs

I’ve added a SCSS function that, when passed a file path, will inject the file as a data URI. This
is super helpful since it ensures that the cursor images will always be available for the CSS. This
issue currently prevents the npm version of the library from being usable at all.

scripts/scssFunctions.js

@@ -1,7 +1,9 @@
 const { types } = require('node-sass'); /* eslint-disable-line import/no-extraneous-dependencies */
 
 const getBuildData = require('./getBuildData');
+const getFileAsDataURI = require('./getFileAsDataURI');
 
 module.exports = {
   'build-data()': () => types.String(getBuildData(true)),
+  'get-file-as-data-uri($filepath)': filepath => types.String(getFileAsDataURI(filepath)),
 };