[FEATURE]: Allow binding the "Switch model variant" function to a key

[ariane-emory]

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

The function listed in the command palette as "Select model variant" ought be able to bound to a key by the user in their opencode.jsonc if so desired.

[github-actions]

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