NOTE: As per the investigation (T284796), we will refer to the new feature as **real-time preview** since there is an existing feature already called [[ https://www.mediawiki.org/wiki/Manual:Live_preview | live preview ]] (which we will later rename)
As a user/developer, I want to add the basic code needed to make real time previews work. This is a small, piecemeal chunk of the project that by itself is not intended to be user-facing. For now, it will depend on the "Show previews without reloading the page" preference (aka "live preview") being set. We'll introduce a feature flag that when enabled will make the preview area continually update as the user is typing. No UI changes are involved, yet.
**Acceptance criteria**
* Add a feature flag for real-time previews. In the code and release notes, make sure this flag is noted as experimental and shouldn't be enabled by sysadmins.
* Monitor keyboard input and continually update the preview area, similar to [[ https://en.wikipedia.org/wiki/User:TheDJ/ActualLivePreview.js | TheDJ's user script ]].
* Use [[ https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.util-method-debounce | mw.util.debounce() ]] to only update the preview area after the user stops typing.
* The code should be part of the existing [[ https://github.com/wikimedia/mediawiki/blob/81897f50b801216b9884051dc715e251c7e029ee/resources/src/mediawiki.action/mediawiki.action.edit.preview.js | mediawiki.action.edit.preview.js ]] module.
* The "real time preview" feature should not show a spinner as the existing live preview feature does.
* The "Preview" button in the UI can stay where it is and function as it does now, for the time being, even though it is redundant for real time previews.
**QA notes**
* Testing can be done on [[ https://patchdemo.wmflabs.org/ | Patch Demo ]] as it will require the feature flag to be enabled (which we won't enable until we're more feature-complete).