Transition to using PluginLogger for logging to a separate file

[helin24]

There's an internal doc on this topic, but the goal is to move our logs to a separate file, flutter.log. This should hopefully:

• Make logging easier to see and understand during development
• Make logging more useful for debugging user issues that are reported to us but hard to reproduce

Original change to add file: #8253

Migrating files that use logs:

• [logging] Change JX Browser file management to plugin logger #8320
• [logging] Add logging to project initialization #8322
• [logging] Use separate log for FlutterCommand #8339, Clean up command log #8341
• Further migrations will probably just refer to this issue.