fix(forms): replace encoded background images with inline SVGs

[jzempel]

Description

These changes address two important concerns:

1. Mitigate radio, checkbox, and toggle mark visibility problems in Windows hi-contrast mode
2. Eradicate all remaining usage of inline data URLs as background images in react-components – alleviating the potential need for img-src 'self' data: CSP (Content Security Policy) flags in order to use Garden components

Detail

• The @zendeskgarden/svg-icons update is needed by forms for this PR, but was updated across all the packages for consistency
• The demo is pre-published to https://amazing-poincare.netlify.com/forms/#basic for review.

Radios, checkboxes, and toggles should look/work exactly the same as https://garden.zendesk.com/react-components/forms/#basic. The only difference is that now they are implemented using inline SVGs instead of encoded SVG background images.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11