External Libraries: Update Underscore to version 1.12.1.

Full set of changes at https://github.com/jashkenas/underscore/compare/1.8.3...1.12.1.  
The new version includes a `sourceMappingURL` that causes a build failure, so this change 
also introduces a task to remove this from the source during the build.

Props mukesh27, hareesh-pillai, desrosj, SergeyBiryukov, TimoTijhof.
Fixes #45785.

git-svn-id: https://develop.svn.wordpress.org/trunk@50650 602fd350-edb4-49c9-b593-d223f7449a82

Gruntfile.js

@@ -1126,6 +1126,26 @@ module.exports = function(grunt) {
 						dest: BUILD_DIR + 'wp-includes/'
 					}
 				]
+			},
+			sourceMaps: {
+				options: {
+					patterns: [
+						{
+							match: new RegExp( '//# sourceMappingURL=.*\\s*' ),
+							replacement: ''
+						}
+					]
+				},
+				files: [
+					{
+						expand: true,
+						flatten: true,
+						src: [
+							BUILD_DIR + 'wp-includes/js/underscore.js'
+						],
+						dest: BUILD_DIR + 'wp-includes/js/'
+					}
+				]
 			}
 		},
 		_watch: {
@@ -1569,6 +1589,7 @@ module.exports = function(grunt) {
 				'includes:emoji',
 				'includes:embed',
 				'replace:emojiBannerText',
+				'replace:sourceMaps',
 				'verify:build'
 			] );
 		}

package-lock.json

@@ -23053,9 +23053,9 @@
 			"dev": true
 		},
 		"underscore": {
-			"version": "1.8.3",
-			"resolved": "https://registry.npmjs.org/underscore/-/underscore-1.8.3.tgz",
-			"integrity": "sha1-Tz+1OxBuYJf8+ctBCfKl6b36UCI="
+			"version": "1.12.1",
+			"resolved": "https://registry.npmjs.org/underscore/-/underscore-1.12.1.tgz",
+			"integrity": "sha512-hEQt0+ZLDVUMhebKxL4x1BTtDY7bavVofhZ9KZ4aI26X9SRaE+Y3m83XUL1UP2jn8ynjndwCCpEHdUG+9pP1Tw=="
 		},
 		"underscore.string": {
 			"version": "3.3.5",

package.json

@@ -145,7 +145,7 @@
 		"react": "16.13.1",
 		"react-dom": "16.13.1",
 		"twemoji": "13.0.2",
-		"underscore": "1.8.3",
+		"underscore": "1.12.1",
 		"whatwg-fetch": "3.0.0"
 	},
 	"scripts": {