Instead of requiring a name_output.sh, expect a file called
.app_filename in the macos/Flutter directory containing just the name of
the application. The expectation is that the Xcode build will create
that file with a script.

This is not intended as a long-term solution, but it's a substantial
improvement over name_output.sh:
- name_output.sh required constructing the full build output path; this
  made sense when it was coupled with build.sh, but now that the
  decision for where build output goes lives in flutter_tool, that logic
  should as well.
- Changing the name of the application required also updating
  name_output.sh, which is error-prone. With .app_filename, it can be
  created using $PRODUCT_NAME, which means that the usual way of setting
  the application name will automatically update this flow as well.

Part of #30706