Allow grouping terminal profiles into nested submenus

### Feature request

The terminal profile dropdown becomes difficult to navigate when many custom profiles are configured, such as local shells, SSH connections, tmux/rmux sessions, containers, and cluster nodes.

Please allow terminal profiles to be grouped into nested submenus.

Example configuration:

```jsonc
"terminal.integrated.profiles.linux": {
"bash": {
"path": "/bin/bash",
"group": "Local"
},
"pwsh": {
"path": "/usr/bin/pwsh",
"group": "Local"
},
"rmux-node-master": {
"path": "/path/to/script",
"group": "RMUX"
},
"rmux-node-data": {
"path": "/path/to/script",
"group": "RMUX"
}
}
```

Expected dropdown:

```text
Local >
bash
pwsh

RMUX >
rmux-node-master
rmux-node-data
```

This would keep the terminal dropdown compact and improve navigation for users with many specialized profiles.

Related issues such as #123128 and #130192 improve the terminal dropdown and profile ordering, but do not provide hierarchical grouping.