Build/Test Tools: Generalize the NPM test workflow.

The NPM test GitHub Actions workflow is currently used to verify that `npm install` and `npm run build` will complete successfully on Windows.

This generalizes the workflow and adds testing for Linux and MacOS to confirm that the scripts used to build WordPress also complete successfully in those environments.

Because MacOS workflow runs consume GHA minutes at a 10x rate, it’s added in a separate job in order to perform a more strict check.

This also adds steps to verify that using `npm run build:dev` and `npm run grunt clean`/`npm run grunt clean —dev` run successfully 

Props johnbillion.
Fixes #52658.
Built from https://develop.svn.wordpress.org/trunk@50485


git-svn-id: http://core.svn.wordpress.org/trunk@50098 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-50484';
+$wp_version = '5.8-alpha-50485';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.