Migrate to ESLint flat config and unblock ESLint 10 upgrade

eslint-plugin-import has no release supporting ESLint 10 (peer range
caps at ^9), and ESLint 10 removed legacy .eslintrc.js support
entirely. Replace .eslintrc.js/.eslintignore with a flat eslint.config.js
and swap eslint-plugin-import for the actively maintained
eslint-plugin-import-x fork.

Fix issues the migration surfaced: stale import/no-unresolved disable
comments (old plugin namespace), missing error causes on rethrown
errors and an unused var/binding flagged by new rules in ESLint's
expanded recommended set, and a pre-existing test:lint glob bug where
`./src/**/*.js` wasn't expanding recursively under sh, silently
skipping top-level src files in CI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: diemol

.eslintignore

@@ -62,7 +62,7 @@ tests/typings/dist
 
 build
 /*.js
-!.eslintrc.js
+!eslint.config.js
 !jest.config.js
 !babel.config.js
 .idea/

.eslintrc.js

@@ -0,0 +1,45 @@
+const js = require('@eslint/js');
+const importX = require('eslint-plugin-import-x');
+const prettierConfig = require('eslint-config-prettier');
+const globals = require('globals');
+
+module.exports = [
+  {
+    ignores: [
+      '**/node_modules/**',
+      '**/*.mustache',
+      '**/*.snap',
+      'build/**',
+      '.husky/**',
+      '**/*.html',
+      'coverage/**',
+      '.vscode/**',
+      '**/dist/**',
+    ],
+  },
+  js.configs.recommended,
+  importX.flatConfigs.errors,
+  importX.flatConfigs.warnings,
+  {
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+      globals: {
+        ...globals.node,
+        ...globals.es2015,
+      },
+    },
+    rules: {
+      'import-x/no-unresolved': [2, {commonjs: true, amd: true}],
+    },
+  },
+  {
+    files: ['tests/**/*.js'],
+    languageOptions: {
+      globals: {
+        ...globals.jest,
+      },
+    },
+  },
+  prettierConfig,
+];