replace lesshint with stylelint (go-gitea#7305)

New CSS linter which is much more powerfull than the previous one.
Configuration is default but I had to remove a few rules that were
throwing too many or weird errors.

More importantly, the linter will exit with code 1 on errors so now our
build will fail if the CSS linter fails which should eliminate linter
errors being introduced without notice.

.stylelintrc

@@ -0,0 +1,11 @@
+extends: stylelint-config-standard
+
+rules:
+  block-closing-brace-empty-line-before: null
+  color-hex-length: null
+  comment-empty-line-before: null
+  declaration-empty-line-before: null
+  indentation: 4
+  no-descending-specificity: null
+  rule-empty-line-before: null
+  selector-pseudo-element-colon-notation: null

Makefile

@@ -392,7 +392,7 @@ js: npm
 
 .PHONY: css
 css: npm
-	npx lesshint public/less/
+	npx stylelint public/less
 	npx lessc --clean-css="--s0 -b" public/less/index.less public/css/index.css
 	$(foreach file, $(filter-out public/less/themes/_base.less, $(wildcard public/less/themes/*)),npx lessc --clean-css="--s0 -b" public/less/themes/$(notdir $(file)) > public/css/theme-$(notdir $(call strip-suffix,$(file))).css;)
 	npx postcss --use autoprefixer --no-map --replace public/css/*