Build/Test Tools: Only include active, stable routes in build.

The `registry.php` file within the built assets from the `gutenberg` repository contains an accurate list of active, stable routes. However, the `build/routes/` directory has the JavaScript and PHP files for all routes, regardless of their status.

This makes adjustments to the `grunt copy` tasks responsible for copying these files into the appropriate locations to extract the list of routes specified in the `registry.php` file so that only the required files are copied.

See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@62079 602fd350-edb4-49c9-b593-d223f7449a82