Build/Test Tools: Ignore third-party plugins in grunt watch task.

This aims to avoid a `JavaScript heap out of memory` fatal error when running `npm run dev`.

Follow-up to [25001].

Props SirLouen, desrosj, tusharbharti, ankitkumarshah, SergeyBiryukov.
Fixes #63606.

git-svn-id: https://develop.svn.wordpress.org/trunk@61475 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

Gruntfile.js

@@ -1299,7 +1299,9 @@ module.exports = function(grunt) {
 					SOURCE_DIR + '**',
 					'!' + SOURCE_DIR + 'js/**/*.js',
 					// Ignore version control directories.
-					'!' + SOURCE_DIR + '**/.{svn,git}/**'
+					'!' + SOURCE_DIR + '**/.{svn,git}/**',
+					// Ignore third-party plugins.
+					'!' + SOURCE_DIR + 'wp-content/plugins/**'
 				],
 				tasks: ['clean:dynamic', 'copy:dynamic'],
 				options: {