Move code editor into example

[romanbsd]

Summary

Moves the code editor widget out of the published package API and into the example app, where it is currently used.

Why

The editor implementation is demo UI and owns rich clipboard behavior. Keeping it in lib/ forces package consumers that only need syntax highlighting to depend on super_clipboard.

Changes

• Move lib/src/code_editor.dart to example/lib/code_editor.dart.
• Stop exporting the editor from package:syntax_highlight.
• Move super_clipboard from root package dependencies to the example app.
• Keep the example app wired to the moved local editor implementation.

Validation

• flutter analyze