Command palette basic items

What does this MR do and why?

Extends search modal with command palette functionality (POC here !121709 (closed))

• type > for some basic create commands (try searching by some keywords - new, create).

Those commands are copied from the Create menu from the sidebar. They were the fastest thing to implement and kind of make sense as commands.

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 scoped search by @user or #issue from the POC

👣 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

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