Update "Clear" button CSS styles for improved accessibility

[ZachSaucier]

The way that the "Clear" button currently functions, there's arguably two issues:

1. The click area to the left of the text is very small (2px).
2. When the button is navigated to via keyboard, the border makes the element look awkward:
   

Both of these issues would be fixed by changing the padding of .pagefind-ui__search-clear from 0 calc(15px * var(--pagefind-ui-scale)) 0 calc(2px * var(--pagefind-ui-scale)) to 0 calc(15px * var(--pagefind-ui-scale)).

Thanks!

[bglw]

Good notes, thanks! Converted to issue for dev.

[ZachSaucier]

Issue for reference: #537