Indicate key submap/group/mode

### Feature Description

Key submaps/groups/modes let you toggle between different sets of keybinds, similar to normal/insert/visual mode in vim etc.
Hyprland supports submaps and a submap is even included in the default config.
MangoWM has an equivalent in "key modes"
unsure about other compositors

It would be nice if DMS had options to show:
- the active submap in bar
- an ODS on submap change

### Use Case

Being in a different submap than you think you are (either because you pressed the wrong button, walked away from the computer for a bit, or just have a particularly complex submap setup) can just be pretty confusing, making it seem like your keys dont work or accidentally running stuff you didn't intend to.
Basically the same reasons as for capslock indicator existing but even moreso.

### Compositor(s)

Hyprland

### If Other, please specify

_No response_

### Proposed Solution

hyprland exposes active submap on socket2 under "submap"
mangoWM lets you watch for changes with `mmsg -g -b`

### Alternatives/Existing Solutions

There is a DMS plugin that shows the active submap on the bar for Hyprland
However, this doesn't have an option to show an OSD on change, and only supports Hyprland.
And subjectively, I feel this is "core feature" enough to belong in base DMS. Waybar also supports it out of the box.