Improve the search bar's clear button

[loic-sharma]

pub.dev's search bar has an 'x' button on the right to clear the current query:

Some potential improvements for this button:

1. It's hard to see in dark mode. Perhaps it should be lighter to increase the color contrast.
2. You can't focus it by pressing tab. This isn't ideal for accessibility
3. When you hover over the button, the cursor should switch to pointer to indicate the x is clickable. Currently, the cursor stays the default pointer cursor.