[gen_l10n] Warn users when placeholder types are converted to 'num' when using pluralization (#108036)

* init

* asdf

* fix tests

* change warning slightly

* fix test again

* fix spacing and comment

* fix lint

This reverts commit 16f44118.

This reverts commit 5fd259be.

* Remove need for unused import for placeholder braces

* Remove need for unused intl import for when plurals aren't used in the generated code

This reverts commit 0cbe5975.

* opt out the flutter tool

* oops EOF

* fix import

* Update tool_backend.dart

* Update daemon_client.dart

* fix more

Instead of using package:package_config to write the .dart_tool/package_config the original json is modified and rewritten.

The .dart_tool/package_config.json file is read twice to simplify control flow.

This also avoids the issue of package:package_config writing directly to local filesystem.

* Reorganize files

* Use l10n.yaml instead of command line args