Command palette user-scope search

What does this MR do and why?

Extends search modal with command palette (search in scope of users specifically) (POC here !121709 (closed))

• type > for some basic create commands (try searching by some keywords - new, create) - added in scope of !121932 (merged)
• type @ to search for users - current MR

Note that so far we do not have distinct requirements so I've implemented it as I thought would be reasonable. Feel free to add ideas what could be the next commands in the command palette which you'd like to have access to.

My plan is:

👣 to extract the #issue search from the POC

👣 implement scoped search inside the project/group

👣 some basic commands context-based - e.g. closing the issue on the issue page or promoting it to an epic. Or some ideas from &8912. Something that would be an easy win

This is an experimental functionality and is the subject of future change or complete removal - we'll get the UX input after a couple of basic experimental commands are added

Screenshots or screen recordings

How to set up and validate locally

👣 Enabled the feature flag

rails c 
Feature.enable(:command_palette)

👣 Click on the Global Search icon in the new navigation

Related to &8912