[Focus] Add run key command to dump the focus tree

[loic-sharma]

Adds a Flutter run key to dump the focus tree: once the app is launched using flutter run, pressing f dumps the focus tree. This aligns with how pressing w dumps the widget hierarchy and t dumps the render tree.

This makes it easier to debug focus issues like:

• What is focused currently?
• Why is tabbing moving my focus unexpectedly?
• Why isn't my shortcut's action running?

Example output:

> flutter run lib\empty.dart -d windows
Launching lib\empty.dart on Windows in debug mode...
Building Windows application...                                     5.4s
✓  Built build\windows\runner\Debug\ui.exe.
Syncing files to device Windows...                                  66ms

Flutter run key commands.
r Hot reload. 🔥🔥🔥
R Hot restart.
h List all available interactive commands.
d Detach (terminate "flutter run" but leave application running).
c Clear the screen
q Quit (terminate the application on the device).

A Dart VM Service on Windows is available at: http://127.0.0.1:49880/aAvLj-OYB2c=/
The Flutter DevTools debugger and profiler on Windows is available at:
http://127.0.0.1:9102?uri=http://127.0.0.1:49880/aAvLj-OYB2c=/

Then when you press f:

FocusManager#59e1c
 └─rootScope: FocusScopeNode#8b67c(Root Focus Scope)

Part of #118536

Screen recording

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[gspencergoog]

Looks great, thanks!

[zanderso]

LGTM