Add feature to continue indented lines in markdown editor

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What does this MR do and why?

Add feature to continue indented lines in markdown editor

This MR adds a new WIP feature flag called continue_indented_text.

The markdown editor uses this flag to allow for enter/return keypresses triggering an auto-indenting behaviour.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screen_Recording_2025-01-29_at_12.02.25_PM

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:continue_indented_text)

2. Visit any wiki
3. Start editing a page

Related to #23978 (closed)