Support custom vim insert-mode mappings (e.g., jj to enter normal mode)

Feature Request

Is your feature request related to a problem?

When using vim mode (/vim), there's no way to configure custom insert-mode mappings. Many vim users rely on inoremap jj <Esc> (or similar mappings like jk) to quickly exit insert mode without reaching for the Escape key. This is one of the most common vim customizations.

Currently, the keybindings system operates at the component level (chat:submit, chat:cancel, etc.) and doesn't have actions for vim-mode operations like entering normal mode. Attempting to use a chord binding like "j j": "chat:cancel" as a workaround breaks normal j typing and triggers the wrong action.

Describe the solution you'd like

Support for custom vim keymaps in vim mode, either through:

1. A vim-specific section in keybindings.json with actions like vim:enterNormalMode
2. Support for a .vimrc-style config for Claude Code's vim mode (e.g., inoremap jj <Esc>)
3. Passing custom keymaps to the underlying vim mode implementation

Describe alternatives you've considered

• Using Escape to enter normal mode (works but is less ergonomic, which is why jj mappings are so popular)
• Using the chord keybinding "j j": "chat:cancel" — this intercepts the first j keypress entirely, preventing normal typing of the letter j

Additional context

This is a very common vim customization — jj, jk, and kj are among the most popular insert-mode escape mappings in the vim community.