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

experimental/context_menus/lib/is_valid_email.dart

@@ -0,0 +1,10 @@
+/// Returns true if the given String is a valid email address.
+bool isValidEmail(String text) {
+  return RegExp(
+    r'(?<name>[a-zA-Z0-9]+)'
+    r'@'
+    r'(?<domain>[a-zA-Z0-9]+)'
+    r'\.'
+    r'(?<topLevelDomain>[a-zA-Z0-9]+)',
+  ).hasMatch(text);
+}