chore: update to eslint flat config & bump to eslint v9

eslint.config.js

@@ -0,0 +1,54 @@
+const globals = require('globals');
+const html = require('eslint-plugin-html');
+const stylistic = require('@stylistic/eslint-plugin-js');
+
+module.exports = [
+    {
+        languageOptions: {
+            globals: {
+                ...globals.browser,
+                L: true,
+                module: false,
+                define: false,
+                require: true,
+            },
+
+            ecmaVersion: 6,
+            sourceType: 'script',
+        },
+        plugins: {
+            '@stylistic/js': stylistic,
+        },
+        rules: {
+            camelcase: 2,
+            '@stylistic/js/quotes': [2, 'single', 'avoid-escape'],
+            '@stylistic/js/no-mixed-spaces-and-tabs': [2, 'smart-tabs'],
+            '@stylistic/js/space-before-function-paren': 2,
+            '@stylistic/js/space-in-parens': 2,
+            '@stylistic/js/object-curly-spacing': 2,
+            '@stylistic/js/array-bracket-spacing': 2,
+            '@stylistic/js/computed-property-spacing': 2,
+            '@stylistic/js/space-before-blocks': 2,
+            '@stylistic/js/keyword-spacing': 2,
+            'no-lonely-if': 2,
+            '@stylistic/js/comma-style': 2,
+            'no-underscore-dangle': 0,
+            'no-constant-condition': 0,
+            '@stylistic/js/no-multi-spaces': 0,
+            strict: 0,
+            '@stylistic/js/key-spacing': 0,
+            'no-shadow': 0,
+            'no-unused-vars': 2,
+            eqeqeq: 2,
+        },
+    },
+    {
+        files: ['**/*.html'],
+        plugins: {
+            html,
+        },
+        settings: {
+            'html/report-bad-indent': 2,
+        },
+    }
+];