Updates `Button` Astro Component

[Ben-Pusey-Bentley]

Changes

Updates the Button Astro Component with various changes requested in the comments of this PR. These changes include:

• Renaming the active prop to isActive
• Creating an IconButton component and moving the shift prop onto that component
• Removing the iuiType prop. The idea value has been added to variant, while dropdown has been made a separate boolean prop
• Adds type='button' to both the button components
• Inserts a span into the Button component for holding button text
• Uses named slots in both of the button components so that button icons no longer need the iui-button-icon class in order to be styled correctly
• Adds iui-button-base class to button components

Testing

Ran css visual tests to make sure that there were no unintended visual changes.

Docs

N/A