fix(buttons): improve stroke/fill icon swap for ToggleIconButton

[jzempel]

Description

• We may need to ease the opacity transition between stroke/fill buttons on toggle.
• Removes invalid theme attribute from the cloned icon.

Detail

@ginnywood please have a look at the "Toggle Button" example in the PR deployment. Note the affect of the various knobs (i.e. "Primary") on the transition animation.

Will impact or be impacted by #889.

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
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage increased (+0.003%) to 95.844% when pulling c69c739 on jzempel/toggle-icon into aefdef3 on main.

[ginnywood]

Looks awesome.

[austingreendev]

@hzhu does this new example need to be included with the storybook PR for react-buttons?