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/README.md

@@ -0,0 +1,55 @@
+# @mattermost/eslint-plugin
+
+An ESLint plugin containing the configuration used by Mattermost as well as support for custom rules specific to the Mattermost code base.
+
+## Custom Rules
+
+### no-dispatch-getstate
+
+Prevents passing a [redux](https://redux.js.org/) store's `getState` into its `dispatch` as an unnecessary second argument.
+
+We started doing this accidentally at some point because of a misunderstanding about how [redux-thunk](https://github.com/reduxjs/redux-thunk) worked, so this stops anyone from making that same mistake again.
+
+Examples of **incorrect** code for this rule:
+```javascript
+export function someAction() {
+    return (dispatch, getState) => {
+        dispatch(doSomething(), getState);
+    };
+}
+```
+
+Examples of **correct** code for this rule:
+```javascript
+export function someAction() {
+    return (dispatch) => {
+        dispatch(doSomething());
+    };
+}
+```
+
+### use-external-link
+
+Ensures that any link which opens a URL outside of Mattermost using `target="_blank"` uses the `ExternalLink` component.
+
+Examples of **incorrect** code for this rule:
+```javascript
+export function SomeLink() {
+    return (
+        <a
+            href="https://example.com"
+            target="_blank"
+            rel="noopener noreferrer"
+        />
+    );
+}
+```
+
+Examples of **correct** code for this rule:
+```javascript
+import ExternalLink from 'components/external_link';
+
+export function SomeLink() {
+    return <ExternalLink href="https://example.com"/>;
+}
+```