Shrink hello-world bundle from 251 KB to 173 KB (-31%) via bundler hints

Two purely-additive bundler hints that let consumer bundlers
(vite/rolldown, webpack, esbuild) tree-shake aggressively through
ember-source's internal barrel re-exports without requiring any
source-file restructuring.

1. **`"sideEffects": false` on `ember-source/package.json`** -
   declares that no module in this package has top-level side
   effects that need to be preserved if the module's exports
   are unused. The bundler can then DCE re-exports through
   `index.ts` barrels that currently anchor the rest of the
   graph in place.

   This is safe in practice because rollup's chunking groups
   symbols with their side effects: any chunk containing the
   classic `Component` class also contains the
   `setInternalComponentManager(CURLY_COMPONENT_MANAGER, Component)`
   call, the `setHelperManager` registrations live in the chunk
   that holds `helper.ts`, etc. Importing a symbol from a chunk
   pulls the chunk's side effects along; apps that don't reach
   those symbols don't need their side effects either.

2. **`treeshake.moduleSideEffects` callback in `rollup.config.mjs`** -
   the package-level `sideEffects: false` declarations on
   `@glimmer/debug`, `@glimmer/debug-util`, and
   `@glimmer/local-debug-flags` get lost when rolldown emits
   shared chunks (debug code from these packages can leak into
   chunks that the renderer-only path then pulls in). The callback
   re-asserts module purity at the chunk level so leaked debug
   code drops out of the renderer-only path.

Measured against `smoke-tests/v2-app-hello-world-template`:

| | raw | gzip |
| - | - | - |
| before | 251.05 KB | 79.75 KB |
| after  | 172.99 KB | 55.31 KB |

Classic `v2-app-template` and v1 `app-template` smoke tests still
build and pass. `pnpm test:node` 20/20. `pnpm vite build --mode
development` (full dev test suite app) builds clean.

Author: NullVoxPopuli

package.json

@@ -16,6 +16,7 @@
     },
     "./package.json": "./package.json"
   },
+  "sideEffects": false,
   "homepage": "https://emberjs.com/",
   "bugs": {
     "url": "https://github.com/emberjs/ember.js/issues"