Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

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);
+}