Generates the PhoenixAssetPipeline manifest.
In :prod, this task writes a precompiled manifest BEAM module. In :dev,
it writes the cached manifest under the configured manifest cache directory.
Class calls in functions and HEEx resolve the current manifest at runtime.
Module-scope calls use mappings prepared before Elixir compilation, so
generating a manifest does not recompile consumer modules.
mix phoenix_asset_pipeline.manifest