Build tools: Build webpack dev in grunt build.

For the WordPress zip we need both the minified and non-minified files. This is because of `SCRIPT_DEBUG`. So in grunt build we need to do both `grunt webpack:prod` and `grunt webpack:dev`.

Props bordoni, atimmer.

Merges [43817] to trunk.

Fixes #45156.
Built from https://develop.svn.wordpress.org/trunk@44161


git-svn-id: https://core.svn.wordpress.org/trunk@43991 1a063a9b-81f0-0310-95a4-ce76da25c4cd