Build/Test Tools: Copy block editor files to `src` after download.

The block editor-related files were previously present in the `src` directory and subject to version control. However, [61439] removed them from version control and has broken some workflows and testing setups that run the PHPUnit tests from `src`.	

There is work ongoing to restore those files to version control with their respective change history. But until that happens, this updates the `download.js` script and `gutenberg:download` Grunt task to run `build:gutenberg —dev` every time the build asset is downloaded to ensure the files are present in `src`.

Props mywp459, amykamala, jorbin.
Fixes #64716. See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@62052 602fd350-edb4-49c9-b593-d223f7449a82