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/windows/.gitignore

@@ -0,0 +1,17 @@
+flutter/ephemeral/
+
+# Visual Studio user-specific files.
+*.suo
+*.user
+*.userosscache
+*.sln.docstates
+
+# Visual Studio build-related files.
+x64/
+x86/
+
+# Visual Studio cache files
+# files ending in .cache can be ignored
+*.[Cc]ache
+# but keep track of directories ending in .cache
+!*.[Cc]ache/