fix: update start and end slot to remove fixed width and height

[khamudom]

Description

On buttons and menu-item, the start and end slots have fixed width and height. This will cause elements larger than 16px to clip.
This PR removes the fixed dimension on the start and end slot but keeps the 16px on the SVG.
I also updated the grid-template-columns property on the menu-item that will also cause clipping with elements that are larger than 42px.
This should not break any current implementation when using a button, anchor, or the menu-item. This will now allow authors to add larger elements to the start and end slots if the design calls for it.

fixes #3904

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.