Refactoring playground (#9426)
* Use @gradio/code.BaseCode instead of its default export like https://github.com/gradio-app/gradio/pull/8804
* Delete unused code
* add changeset
* Fix
* Rename a variable to be descriptive
* Mount single <Code> instance instead of creating one for each demo
* Fix the initial value passed to createGradioApp
* Use const instead of let
* Rename variable
* Update
* Fix layout
* Restore the preset requirements
* Delete unused variable
* Add type hint
* Attach the keydown handler directly to the input element instead of the window object
* Add code editor widget
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