[flutter_tools] Fix crash when migrating flow-style exclude lists in analysis_options.yaml (#191269)

## Description

Avoids a crash in the `yaml_edit` package when migrating
`analysis_options.yaml` where the `exclude` list is formatted in flow
style (e.g. `[ ... ]`) and contains internal newlines and trailing
commas.

The `yaml_edit` package (version 2.2.4) has a bug when appending to such
lists via `appendToList`, which results in invalid YAML generation and
triggers a `_YamlAssertionError` (tracked upstream in
https://github.com/dart-lang/tools/issues/2532).

To work around this, `AnalysisOptionsMigration` checks if the `exclude`
list's style is `CollectionStyle.FLOW`. If it is, it updates the entire
list using `editor.update` instead of appending. This converts the list
to block style (which is preferred in `analysis_options.yaml`) and
avoids the crash. Invalid types within the list are preserved for
consistency with block-style migration.

## Related Issues
Fixes https://github.com/flutter/flutter/issues/191060
Upstream issue: https://github.com/dart-lang/tools/issues/2532

## Tests
- Added `migrates and merges excludes when list is in flow style with
newlines` to
`packages/flutter_tools/test/general.shard/migrations/analysis_options_migration_test.dart`.