Move the shared ESLint plugin/config into the monorepo (#24670)

* Move the shared ESLint plugin/config into the monorepo

* Add peer dependencies to ESLint plugin

webapp/platform/eslint-plugin/rules/use-external-link.js

@@ -0,0 +1,43 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+const astUtils = require('jsx-ast-utils')
+const getElementType = require('eslint-plugin-jsx-a11y/lib/util/getElementType')
+
+module.exports =  {
+    meta: {
+        docs: {
+            description: 'Enforce all anchors with target="_blank" to use ExternalLink component',
+        },
+    },
+    create: (context) => {
+        const elementType = getElementType(context);
+        return {
+            JSXOpeningElement: (node) => {
+                const { attributes } = node;
+                const typeCheck = 'a';
+                const nodeType = elementType(node);
+                // Only check anchor elements
+                if (!nodeType || typeCheck !== nodeType) {
+                    return;
+                }
+
+                const propsToValidate = ['target'];
+                const values = propsToValidate.map((prop) => astUtils.getPropValue(astUtils.getProp(node.attributes, prop)));
+                // Checks if the target attribute is set to _blank (ie, is an external link)
+                const hasBlankTarget = values.some((value) => value != null && value === '_blank');
+
+                // When there is no target value at all, this rule does not apply:
+                if (!hasBlankTarget) {
+                    return;
+                }
+
+                context.report({
+                    node,
+                    message: 'Use ExternalLink component (components/external_link) for _blank target link-outs',
+                });
+                return
+            },
+        };
+    },
+};