build: add stylelint rule to prevent usage of /deep/ (#4841)

* build: add stylelint rule to prevent usage of /deep/

Adds a custom Stylelint rule to prevent usages of `/deep/` inside selectors.

Relates #4726.

* fix: address feedback

stylelint-config.json

@@ -1,10 +1,12 @@
 {
   "plugins": [
     "./tools/stylelint/no-prefixes/no-prefixes.js",
-    "./tools/stylelint/selector-nested-pattern-scoped/index.js"
+    "./tools/stylelint/selector-nested-pattern-scoped/index.js",
+    "./tools/stylelint/selector-no-deep/index.js"
   ],
   "rules": {
     "material/no-prefixes": [["last 2 versions", "not ie <= 10", "not ie_mob <= 10"]],
+    "material/selector-no-deep": true,
     "material/selector-nested-pattern-scoped": [".*[^&]$", {
       "message": "The & operator is not allowed at the end of theme selectors.",
       "filePattern": "-theme\\.scss$"

tools/stylelint/selector-no-deep/index.js

@@ -0,0 +1,37 @@
+const stylelint = require('stylelint');
+const isStandardSyntaxRule = require('stylelint/lib/utils/isStandardSyntaxRule');
+const isStandardSyntaxSelector = require('stylelint/lib/utils/isStandardSyntaxSelector');
+
+const ruleName = 'material/selector-no-deep';
+const messages = stylelint.utils.ruleMessages(ruleName, {
+  expected: selector => `Usage of the /deep/ in "${selector}" is not allowed`,
+});
+
+
+/**
+ * Stylelint plugin that prevents uses of /deep/ in selectors.
+ */
+const plugin = stylelint.createPlugin(ruleName, isEnabled => {
+  return (root, result) => {
+    if (!isEnabled) return;
+
+    root.walkRules(rule => {
+      if (rule.parent.type === 'rule' &&
+          isStandardSyntaxRule(rule) &&
+          isStandardSyntaxSelector(rule.selector) &&
+          rule.selector.includes('/deep/')) {
+
+        stylelint.utils.report({
+          result,
+          ruleName,
+          message: messages.expected(rule.selector),
+          node: rule
+        });
+      }
+    });
+  };
+});
+
+plugin.ruleName = ruleName;
+plugin.messages = messages;
+module.exports = plugin;