Add FloatingActionButtonLocation.startContained

### Use case

First of all, thank you for your amazing work on the Flutter framework.

Flutter already has left-aligned FAB locations like `startDocked`, `startFloat`, and `startTop`. However, it is missing `startContained` to position the FAB on the left side inside a BottomAppBar.
https://api.flutter.dev/flutter/material/FloatingActionButtonLocation-class.html

Since `endContained` exists for the right side, it would be great to have `startContained` for the left side to support left-handed use cases and custom layouts without needing to write custom SafeArea/padding calculation classes.

### Proposal

Please add `FloatingActionButtonLocation.startContained` that mirrors the behavior of endContained on the start side.

Thank you for your time and consideration.