Build/Test Tools: Test the Gutenberg plugin build process.

A very common contributor setup is having a copy of the `gutenberg` development repository within a checkout of the `wordpress-develop` repository. On occasion, there are some strange incompatibilities that come up when using this setup. A few examples can be seen in #58671 and #59634.

This changeset helps ensure that these edge cases are not introduced by testing the Gutenberg plugin’s build process within WordPress configured to run from both the `src` and `build` directories.

This also renames the “Test npm” workflow to a more general “Test Build Processes”, which more accurately describes what is actually being tested within it and allows these new test jobs to be grouped in.

And finally, the logic within the workflow has been split out into two callable workflows. This helps avoid code duplication within the workflow, and allows for better grouping on the workflow run screen.

Props swissspidy, aferica, SergeyBiryukov, antonvlasenko, desrosj.
Fixes #59632. See #58671, #59634.
Built from https://develop.svn.wordpress.org/trunk@56976


git-svn-id: http://core.svn.wordpress.org/trunk@56487 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-56975';
+$wp_version = '6.5-alpha-56976';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.