pw_console: HelpWindow for displaying keybindings

No-Docs-Update-Reason: prompt_toolkit UI boilerplate
Change-Id: I61cae3370390596cffe25e91640666457f0bfd2f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48802
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>
Pigweed-Auto-Submit: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Joe Ethier <jethier@google.com>
diff --git a/pw_console/py/BUILD.gn b/pw_console/py/BUILD.gn
index e6b2848..d680a49 100644
--- a/pw_console/py/BUILD.gn
+++ b/pw_console/py/BUILD.gn
@@ -22,10 +22,14 @@
     "pw_console/__init__.py",
     "pw_console/__main__.py",
     "pw_console/console_app.py",
+    "pw_console/help_window.py",
     "pw_console/key_bindings.py",
     "pw_console/style.py",
   ]
-  tests = [ "console_app_test.py" ]
+  tests = [
+    "console_app_test.py",
+    "help_window_test.py",
+  ]
   python_deps = [
     "$dir_pw_cli/py",
     "$dir_pw_tokenizer/py",