Cleans old JS, CSS and Webpack files from /src so they are not automatically copied to /build when running `grunt build`. Fixes errors that may be caused by copying outdated files and/or directories to /build.
Props: desrosj, isabel_brison, SergeyBiryukov, ironprogrammer, mukesh27, robinwpdeveloper, razthee007, costdev, peterwilsoncc, azaozz.
Fixes: #47749.
Built from https://develop.svn.wordpress.org/trunk@55484
git-svn-id: http://core.svn.wordpress.org/trunk@55017 1a063a9b-81f0-0310-95a4-ce76da25c4cd