[FEATURE]: Support multi-letter keybindings after leader key (e.g., `<leader>bb`)

[yuyudhan]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently, only single-letter keybindings work after the leader key. Multi-letter sequences exit leader mode after the first keypress.

Example

"sidebar_toggle": "<leader>b"   // Works
"sidebar_toggle": "<leader>bb"  // Doesn't work - exits leader mode after first 'b'

Expected behavior

Pressing <leader>bb should trigger sidebar_toggle.

Actual behavior

Pressing <leader>b exits leader mode immediately; the second b is typed as input.

Feature request

1. Support multi-letter key sequences after leader (e.g., <leader>bb, <leader>gc)
2. Consider adding a tmux-style mode system where leader activates a "command mode" that persists until an action is matched or escaped

This would enable vim/tmux-like keybinding namespaces (e.g., <leader>g for git commands, <leader>b for buffer/sidebar commands).

[github-actions[bot]]

For keybind-related issues, please also check our pinned keybinds documentation: #4997