Login and Registration: Fix password icon alignment.

Change the line-height of the `button` element for toggling password visibility to center the icon vertically.

Props wildworks, niravsherasiya7707, khushi1501, joedolson.
Fixes #64869.

git-svn-id: https://develop.svn.wordpress.org/trunk@62038 602fd350-edb4-49c9-b593-d223f7449a82