Build/Test Tools: Move icon library SVG files to `wp-includes/images`.

This makes several changes to the build script to place the icon library files into more appropriate locations.
- The icon library SVG files are now copied into the `wp-includes/images/icon-library` directory instead of `wp-includes/icons`.
- The name of the `manifest.php` file has changed to `icon-library-manifest.php` and is now copied to `wp-includes/assets`.
- An unnecessary `trailingslashit()` has been removed.
- The `gutenberg-` prefix for the corresponding `grunt copy` tasks has been removed.

These values are merged into the `script-loader-packages.php` file during a `build`, so the original PHP files are not required.

Props mcsf, desrosj.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@62077 602fd350-edb4-49c9-b593-d223f7449a82