fix: correct anchor and button stylesheets

[nicholasrice]

Description

Several issues outlined in #3269 were making styling buttons and anchors difficult. While there still exists some complexity in styling padding / borders, this PR aims to reduce the styling edge-cases that arise from the implementation details of both fast-button and fast-anchor.

Motivation & context

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[chrisdholt]

Clever solution with the transparent bg on the control itself 👍