Add ESLint Rule to Signals Plugin

Signed-off-by: Aditya Kumar <aditya.kumar60@infosys.com>

.changeset/unlucky-drinks-glow.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-signals': patch
+---
+
+Added ESLint rule `no-top-level-material-ui-4-imports` in the `signals` plugin to migrate the Material UI imports.

plugins/signals/.eslintrc.js

@@ -1 +1,5 @@
-module.exports = require('@backstage/cli/config/eslint-factory')(__dirname);
+module.exports = require('@backstage/cli/config/eslint-factory')(__dirname, {
+    rules: {
+      '@backstage/no-top-level-material-ui-4-imports': 'error',
+    },
+});

plugins/signals/dev/index.tsx

@@ -17,7 +17,7 @@ import React from 'react';
 import { createDevApp } from '@backstage/dev-utils';
 import { signalsPlugin } from '../src/plugin';
 import { Content, Header, Page } from '@backstage/core-components';
-import { Typography } from '@material-ui/core';
+import Typography from '@material-ui/core/Typography';
 
 createDevApp()
   .registerPlugin(signalsPlugin)