Fixes AdvancedMarker anchor handling in `google_maps_flutter_web`.

Previously `AdvancedMarker.anchor` was not applied on web, so custom advanced marker content was positioned as if it used the default anchor. This caused markers with non-default anchors to appear offset from their intended map coordinates.

This PR converts Flutter marker anchor offsets into the Google Maps JavaScript Advanced Marker `anchorLeft` and `anchorTop` CSS offset properties when creating marker options, and reapplies those properties when an existing advanced marker is updated.

Adds integration test coverage for creating and updating an `AdvancedMarker` with custom anchors.

Addresses flutter/flutter#80578.

## Pre-Review Checklist