Find and replace for markdown editors

What does this MR do and why?

Implements Find and Replace functionality for the Markdown Editor

This is the initial commit of a commit-series for implementing the find and replace functionality to our markdown editors. This commit registers the shortcuts behind a feature flag, and displays a simple input on the top-right of the editor whenever CMD+F is pressed on a focused markdown editor.

MR acceptance checklist

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Screenshots or screen recordings

find-and-replace

How to set up and validate locally

1. Enable the feature flag: echo "Feature.enable(:find_and_replace)" | rails c
2. Visit any markdown editor (for instance an issue page)
3. Toggle the edit mode
4. Press CMD+F
5. Expect an input to be shown on top-right of the editor

Things to test:

• CMD+F should open the browser find window when a text editor is not being focused
• CMD+F should open the find and replace bar of the relevant text editor (when there are multiple text editors in the same page)
• CMD+F should open the find and replace bar only when a text editor is being focused
• Escape should close the find and replace bar
• CMD+F shortcut should be registered only when find_and_replace feature flag is turned on

Related to #481521 (closed)