Move govuk-frontend ESLint config to @govuk-frontend/eslint-config

Moves the `.eslintrc.js` file usit `git mv` to keep the history of the file.
Adding back a `.eslintrc.js` file in the same commit to keep ESLint working
would confuse Git, though. Instead, we can take advantage of the `.cjs` extension,
which will be understood just as well by ESLint, but is a different file from Git's perspective.

packages/govuk-frontend/.eslintrc.cjs

@@ -0,0 +1,3 @@
+const createConfig = require('@govuk-frontend/eslint-config/govuk-frontend.eslintrc.js')
+
+module.exports = createConfig(__dirname)

packages/govuk-frontend/package.json

@@ -65,6 +65,7 @@
     "@babel/core": "^7.26.0",
     "@babel/preset-env": "^7.26.0",
     "@govuk-frontend/config": "*",
+    "@govuk-frontend/eslint-config": "*",
     "@govuk-frontend/helpers": "*",
     "@govuk-frontend/lib": "*",
     "@govuk-frontend/tasks": "*",

packages/govuk-frontend/.eslintrc.js → shared/eslint-config/govuk-frontend.eslintrc.js

@@ -1,6 +1,6 @@
 const { join } = require('path')
 
-module.exports = {
+module.exports = (dirname) => ({
   settings: {
     node: {
       version: '^18.12.0'
@@ -14,7 +14,7 @@ module.exports = {
       parserOptions: {
         // Note: Allow ES2015 for import/export syntax
         ecmaVersion: '2015',
-        project: [join(__dirname, 'tsconfig.build.json')]
+        project: [join(dirname, 'tsconfig.build.json')]
       },
       plugins: ['@typescript-eslint', 'es-x'],
       extends: [
@@ -95,4 +95,4 @@ module.exports = {
       }
     }
   ]
-}
+})