Add a command palette to the UI

[NickLanam]

Pixie's UI used to have a command card that could be summoned with Ctrl/Cmd + K. It allowed quickly choosing+running PxL scripts and their arguments, without touching the mouse.

It looked like this:

It was disabled last year because we weren't happy with the quality of its suggestions. Now is a good time to restore it.

This will happen in several stages:

1. Restore basic features Example: press a keyboard shortcut, type script:httpTAB, see script:px/http_data start_time:-5m, press Enter, and run that script with those arguments.

2. Restore advanced features Suggestions for pods, namespaces, and services; pre-filling a command from just a partial argument; tab stops; error hints... Example: type pod:foo; first suggestion changes to script:px/pod pod:foo; starts suggesting pods names matching foo.

3. More kinds of commands After the palette works well enough, add other actions that happen just by pressing Enter on a suggestion. Examples: light → Set theme to light mode; clusters → Go to clusters tab in Admin page