build: Simplify linter configs

* eslint: Already excludes node_modules by default.
  https://eslint.org/docs/user-guide/configuring#ignoring

* eslint: Expands directories by default, much faster to
  let ESLint crawl the directories while knowing what to exclude,
  then to let Grunt expand **/** recursively run each file
  individually.
  This makes Grunt run ESLint the same way as one normally
  would from the command line: `eslint .`, except without
  the eslint CLI sub-process.

* eslint: Preserve the exclude for vendor via eslint config
  so that it also works when running ESLint outside Grunt,
  e.g. in IDEs or from the command line with extra options.

* stylelint: Ignores node_modules by default.
  https://stylelint.io/user-guide/node-api/#files

Change-Id: I1a78e9d93e7666c1539bcf4a3bc50306cecf89ba
(cherry picked from commit 20f34321f4)

Gruntfile.js

@@ -9,11 +9,7 @@ module.exports = function ( grunt ) {
 
 	grunt.initConfig( {
 		eslint: {
-			all: [
-				'**/*.js',
-				'!node_modules/**',
-				'!vendor/**'
-			]
+			all: '.'
 		},
 		stylelint: {
 			options: {
@@ -21,7 +17,6 @@ module.exports = function ( grunt ) {
 			},
 			all: [
 				'**/*.{css,less}',
-				'!node_modules/**',
 				'!vendor/**'
 			]
 		},

package.json

@@ -12,5 +12,8 @@
     "grunt-stylelint": "0.9.0",
     "stylelint": "8.2.0",
     "stylelint-config-wikimedia": "0.4.2"
-  }
+  },
+  "eslintIgnore": [
+    "vendor/**"
+  ]
 }