fix(core): add missing migration to npm package (#29705)

While running `ng update @angular/core --next`, the following error would be displayed:

```
Cannot find module '....\node_modules\@angular\core\schematics\migrations\template-var-assignment\index'
```

This happened because the Schematics migration was referenced, but not included.

This commit fixes that bug by including the migration in the Bazel npm package dependencies.

PR Close #29705

packages/core/schematics/BUILD.bazel

@@ -9,5 +9,8 @@ npm_package(
     name = "npm_package",
     srcs = ["migrations.json"],
     visibility = ["//packages/core:__pkg__"],
-    deps = ["//packages/core/schematics/migrations/static-queries"],
+    deps = [
+        "//packages/core/schematics/migrations/static-queries",
+        "//packages/core/schematics/migrations/template-var-assignment",
+    ],
 )