Editor: Add custom fields meta box support in the block editor.

This brings support for the custom fields meta box into the new block editor.

The `webpack` and `copy-webpack-plugin` packages have also been updated.

This does not bump the `@wordpress` packages like in [43861] because of conflicts with package versions already installed in `trunk`. The packages will be brought up to date in a subsequent merge.

Merges [43861] and [43863] into trunk.

See #45145.
Fixes #45257.
Built from https://develop.svn.wordpress.org/trunk@44260


git-svn-id: https://core.svn.wordpress.org/trunk@44090 1a063a9b-81f0-0310-95a4-ce76da25c4cd