fix(language-service): Use index.d.ts for typings (#33043)

The current `typings` value in `package.json` causes the import of
`@angular/language-service` in TypeScript to be generated as

```
const language_service_1 = require("@angular/language-service/language-service");
```
in CJS output.

This breaks the import shim that overwrites the behavior of `require` at
runtime. Changing the typings to `index.d.ts` fixes the issue.

PR Close #33043

packages/language-service/package.json

@@ -4,7 +4,7 @@
   "description": "Angular - language services",
   "main": "./bundles/language-service.umd.js",
   "module": "./fesm5/language-service.js",
-  "typings": "./language-service.d.ts",
+  "typings": "./index.d.ts",
   "author": "angular",
   "license": "MIT",
   "repository": {