Advanced editor for security policy

What does this MR do and why?

1. New advanced policy editor split view
2. Instead of switching between modes, it displays both modes side to side
3. It can collapse each half, giving more space for either of modes
4. It disables rule mode immediately if yaml mode has invalid yaml
5. User can change width of modes by dragging split view border

References

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• Prototype

MR acceptance checklist

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Screenshots or screen recordings

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Description	UI
Big screen editing policy	editing.mov
Refresh page, width is preserved	saved width.mov
Smaller screen	smaller screen.mov

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable feature flag:

   Feature.enable(:security_policies_split_view)

2. Go to project/group

3. Secure -> Policies

4. New policy -> select any type

Related to #450705