Custom context menu example (#1463)

* flutter create

* Copied in from 'contextual-menu-examples' repo

* Works again with the latest updates to the branch

* Clean up anywhere example

* Clean up other examples

* Add to CI

* Updated with release version of context menus, and added platform switcher

* Generated files

* Home icon on your original platform

* Remove web_dashboard from ci, not sure why that change was there...

* Add context_menu to beta channel, but commented out, for the future

* +x permissions on the master script, which I may have accidentally changed before?

* Actual bash comment

* dart format

* Natural default platform'

* A working test, for the email page

* Import order fix

* Test some pages

* More tests

* dart format

tool/flutter_ci_script_beta.sh

@@ -20,6 +20,9 @@ declare -ar PROJECT_NAMES=(
     "code_sharing/shared"
     "desktop_photo_search/fluent_ui"
     "desktop_photo_search/material"
+    # TODO(justinmc): To be enabled when its PR hits the master channel.
+    # https://github.com/flutter/flutter/pull/107193
+    # "experimental/context_menus"
     "experimental/federated_plugin/federated_plugin"
     "experimental/web_dashboard"
     # TODO(DomesticMouse): Needs to be re-formatted for Flutter beta