Remove gulp-stylelint for direct stylelint integration

While looking at what would be needed for govuk-frontend to adopt a
stylelint 14 compatible release of styleint-config-gds [1] I discovered
that a blocker for that upgrade is the use of gulp-stylelint [2] which
only officially supports Stylelint up to version 13 and doesn't seem to
be actively iterated.

There didn't seem to be much that was lost by removing gulp-stylelint,
it actually made SCSS linting consistent with the approach taken for JS.
As the command to run stylelint is longer than standard I added
additional tasks to the NPM file that are consistent for both JS and
SCSS linting.

[1]: alphagov/stylelint-config-gds#22
[2]: olegskl/gulp-stylelint#132

gulpfile.js

@@ -14,7 +14,6 @@ require('./tasks/gulp/watch.js')
 require('./tasks/gulp/copy-to-destination.js')
 require('./tasks/gulp/asset-version.js')
 require('./tasks/gulp/sassdoc.js')
-require('./tasks/gulp/stylelint.js')
 
 // Umbrella scripts tasks for preview ---
 // Runs js lint and compilation
@@ -24,10 +23,9 @@ gulp.task('scripts', gulp.series(
 ))
 
 // Umbrella styles tasks for preview ----
-// Runs scss lint and compilation
+// Runs scss compilation
 // --------------------------------------
 gulp.task('styles', gulp.series(
-  'scss:lint',
   'scss:compile'
 ))
 
@@ -43,7 +41,6 @@ gulp.task('copy:assets', () => {
 // Runs js, scss and accessibility tests
 // --------------------------------------
 gulp.task('test', gulp.series(
-  'scss:lint',
   'scss:compile'
 ))
 

package.json

@@ -16,10 +16,13 @@
     "pre-release": "node bin/check-nvmrc.js && ./bin/pre-release.sh",
     "build:package": "node bin/check-nvmrc.js && gulp build:package --destination 'package' && npm run test:build:package",
     "build:dist": "node bin/check-nvmrc.js && gulp build:dist --destination 'dist' && npm run test:build:dist",
-    "test": "npm run test:dependencies && standard && gulp test && gulp copy-assets && jest --testPathIgnorePatterns='after-*'",
+    "test": "npm run test:dependencies && npm run lint && gulp test && gulp copy-assets && jest --testPathIgnorePatterns='after-*'",
     "test:dependencies": "npm ls --depth=0",
     "test:build:package": "jest tasks/gulp/__tests__/after-build-package.test.js",
-    "test:build:dist": "jest tasks/gulp/__tests__/after-build-dist.test.js"
+    "test:build:dist": "jest tasks/gulp/__tests__/after-build-dist.test.js",
+    "lint": "npm run lint:js && npm run lint:scss",
+    "lint:js": "standard",
+    "lint:scss": "stylelint app/**/*.scss src/**/*.scss"
   },
   "devDependencies": {
     "acorn": "^7.4.0",
@@ -47,7 +50,6 @@
     "gulp-postcss": "^8.0.0",
     "gulp-rename": "^2.0.0",
     "gulp-sass": "^5.0.0",
-    "gulp-stylelint": "^13.0.0",
     "gulp-task-listing": "^1.1.0",
     "gulp-uglify": "^3.0.2",
     "html5shiv": "^3.7.3",