This PR is intended to upgrade some of the core dependencies ahead of more significant changes coming in the next major release.

• upgrades Babel ^6.26.0 ➡️ ^7.15.0
  • removes babel-plugin-external-helpers in favour of @babel/plugin-transform-runtime
  • this was needed to prevent externalHelpers being used as a global
  • see https://github.com/rollup/plugins/tree/master/packages/babel#babelhelpers
• upgrades Storybook ^3.3.12 ➡️ ^6.3.6
  • all stores re-written in modern args format
  • replaces @storybook/addon-knobs with @storybook/addon-controls
• upgrades Jest ^22.2.1 ➡️ ^27.0.6
• upgrades Rollup ^0.55.3 ➡️ ^2.55.1
  • removes namedExports (no longer required)
  • adds preventAssignment: true to @rollup/plugin-replace options
  • replaces rollup-plugin-uglify-es with rollup-plugin-terser

This PR also addressed the following Security Alerts

#258
#251
#247
#246
#244
#239
#229
#208