[Emotion] EuiButton

[daveyholler]

• [Emotion] Convert EuiButton #6150
• [EuiButtonGroup] Convert to Emotion #6841
• [Emotion] Convert EuiButtonIcon #6844
• [Emotion] Convert EuiButtonEmpty #6863

Individual Emotion checklists should be in each PR.

[github-actions]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.