Un-gitignore modules.ts (#29851)

As of #29089, `modules.ts` is no longer auto-generated, so should not be
gitignored. Indeed, having a copy sitting around in your working copy can
produce unexpected results, and removing it from the ignorelist at least gives
maintainers a hint about what might be going wrong.

.gitignore

@@ -25,7 +25,6 @@ electron/pub
 .env
 /coverage
 # Auto-generated file
-/src/modules.ts
 /src/modules.js
 /build_config.yaml
 /book