Fix trailing comma when adding to flow collections in yaml_edit (#2533)

Fixes https://github.com/dart-lang/tools/issues/2532

### Problem
When appending elements to a flow list (`[...]`) or adding keys to a flow map (`{...}`) that contains a trailing comma, `yaml_edit` unconditionally prepended a comma without checking for the existing trailing comma. This produced invalid double-comma syntax (e.g. `, \n , android/**]`), failing internal YAML parsing validation with an assertion error.

Furthermore, multiline flow collections with trailing commas had their newly appended elements and closing delimiters misaligned on column 0.

### Solution
1. **Trailing Comma Detection**:
   - Added `betweenHasTrailingComma` in `pkgs/yaml_edit/lib/src/utils.dart` to check if a comma exists between the last element and the closing delimiter while ignoring comments (`# ...`).
   - If a trailing comma is present, `_appendToFlowList` and `_addToFlowMap` avoid prepending an extra comma and preserve the trailing comma style.
2. **Multiline Indentation Alignment**:
   - Added `formatMultilineFlowTrailingEntry` in `pkgs/yaml_edit/lib/src/utils.dart` to calculate the indentation difference between collection elements and the closing delimiter.
   - Newly appended items in multiline flow lists and maps are now indented to match preceding entries, with the closing delimiter remaining on its own indented line.
3. **Tests**:
   - Added unit tests covering multiline flow lists/maps with and without trailing commas, comments, and nested structures in `pkgs/yaml_edit/test/append_test.dart` and `pkgs/yaml_edit/test/update_test.dart`.
4. **Changelog**:
   - Updated `pkgs/yaml_edit/CHANGELOG.md`.