Exposes MathKeyboard and adds some design improvements

[tomkuschka]

Description

Exposes MathKeyboard so it can be used independently from MathField. Also adds a hover effect on MathKeyboard for better web support and some padding.

Checklist

Remove If [...] items that do not apply to your PR.

• I have made myself familiar with the CaTeX
  contributing guide.
• I added a PR description.
• I linked all related issues and PRs I could find (no links if there are none).
• If this PR changes anything about the main math_keyboard or example package
  (also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change
  on its own is not worth an update).
• If this PR includes a notable change in the math_keyboard package, I updated the version
  according to Dart's semantic versioning.
• If there is new functionality in code, I added tests covering all my additions.
• All required checks pass.

[vmalyi]

@tomkuschka, what about all those failed checks?

[tomkuschka]

@vmalyi They also failed on previous PRs. I think we can ignore them

[edhom]

LGTM