chore: update all icons to use high-quality .svg files (#165)

## Description

This PR updates all of our current icons to use high-quality SVG files.
All icons use a perfect square aspect ratio to make sure they look good
on the registry.coder.com website, too.

This PR does **not** update our validation process to enforce the use of
.svg files. That may be something worth considering in the future, but
better to wait until we know for certain that we won't ever need other
image formats.

## Type of Change

- [x] Other

## Testing & Validation

- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun run fmt`)
- [x] Changes tested locally

Author: buenos-nachos