website: fixed stylelint dependency

- Fixed version in package.json
- Updated rules to match Go CSS styleguide

Change-Id: Ia41bb9000042e3c23732920e32c3e7f063a13350
Reviewed-on: https://go-review.googlesource.com/c/website/+/377735
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>
Trust: Jamal Carvalho <jamalcarvalho@google.com>

.stylelintrc.yaml

@@ -12,3 +12,4 @@ rules:
   order/properties-alphabetical-order: true
   unit-disallowed-list:
     - 'px'
+  selector-class-pattern: '^[a-zA-Z\-]+$'

package.json

@@ -10,6 +10,7 @@
     "eslint-plugin-prettier": "4.0.0",
     "jest": "27.4.7",
     "prettier": "2.5.1",
+    "stylelint": "14.2.0",
     "stylelint-config-prettier": "9.0.3",
     "stylelint-config-standard": "24.0.0",
     "stylelint-order": "5.0.0",