Type-check JavaScript explicitly in the shared tsconfig

@tsconfig/strictest dropped checkJs in 2.0.8, so every .js and .mjs file
fell out of the program: lint could not parse them with type information,
and JavaScript in every repository built with this config went unchecked.
allowJs and checkJs are now set here, independent of the base.

Two @ts-expect-error lines in config/eslint.js no longer had an error to
expect: eslint-plugin-import 2.32 and eslint-plugin-react 7.37 ship types.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

packages/dev/config/eslint.js

@@ -10,7 +10,6 @@ import standardConfig from 'eslint-config-standard';
 import deprecationPlugin from 'eslint-plugin-deprecation';
 // @ts-expect-error No definition for this one
 import headerPlugin from 'eslint-plugin-header';
-// @ts-expect-error No definition for this one
 import importPlugin from 'eslint-plugin-import';
 // @ts-expect-error No definition for this one
 import importNewlinesPlugin from 'eslint-plugin-import-newlines';
@@ -20,7 +19,6 @@ import jestPlugin from 'eslint-plugin-jest';
 import nPlugin from 'eslint-plugin-n';
 // @ts-expect-error No definition for this one
 import promisePlugin from 'eslint-plugin-promise';
-// @ts-expect-error No definition for this one
 import reactPlugin from 'eslint-plugin-react';
 // @ts-expect-error No definition for this one
 import reactHooksPlugin from 'eslint-plugin-react-hooks';

packages/dev/config/tsconfig.json

@@ -5,6 +5,14 @@
    */
   "extends": "@tsconfig/strictest/tsconfig.json",
   "compilerOptions": {
+    /**
+     * JavaScript sources are type-checked like the rest. The base set this
+     * until @tsconfig/strictest 2.0.8 dropped checkJs, which left every .js and
+     * .mjs file outside the program: lint could not parse them with type info.
+     */
+    "allowJs": true,
+    "checkJs": true,
+
     /**
      * Aligns with packages/dev/scripts/pezkuwi-dev-build-ts & packages/dev-ts/src/loader
      * (target here is specifically tied to the minimum supported Node version)