fix(generators): wire lint+typecheck into pipeline and fix uncovered errors

The turbo/generators package shipped without `lint` or `typecheck` scripts,
so `pnpm lint` / `pnpm typecheck` at the root silently skipped it. This
masked 2 ESLint errors (unused imports) and 11 TypeScript errors (relative
imports missing the `.js` extension required by `moduleResolution: NodeNext`,
plus JSON imports missing the `with { type: "json" }` attribute).

- Add `lint` and `typecheck` scripts to turbo/generators/package.json so the
  turbo pipeline picks them up (lint: 14/14, was 13/13).
- Add `.js` extensions to 7 relative imports across config.test.ts,
  lib/core-package-utils.test.ts, lib/snapshot.test.ts, and the 4 e2e tests.
- Add `with { type: "json" }` attributes to 4 snapshot JSON imports in the
  e2e tests.
- Remove unused `existsSync` and `splicePluginImportsAt` imports from
  lib/core-package-utils.test.ts.
- Declare `@repo/core-typescript` + `typescript` devDependencies so the
  generators package can run `tsc --noEmit` for typecheck.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

turbo/generators/lib/core-package-utils.test.ts

@@ -1,6 +1,5 @@
 import { describe, it, expect } from "vitest";
 import {
-  existsSync,
   mkdtempSync,
   mkdirSync,
   writeFileSync,
@@ -12,10 +11,9 @@ import {
   assertOptionalPackageNotPresent,
   addToTranspilePackages,
   splicePluginRulesAt,
-  splicePluginImportsAt,
   addBoundariesEntry,
   emitTemplateTree,
-} from "./core-package-utils";
+} from "./core-package-utils.js";
 
 describe("assertOptionalPackageNotPresent", () => {
   it("throws if packages/<name>/ exists in cwd", () => {