Add support for creating a new empty buffer (C-k i).

- Introduced `BufferNew` command to create and switch to a new unnamed buffer.
- Registered `BufferNew` in the command registry and updated keymap and help text.
- Implemented `cmd_buffer_new()` to handle buffer creation and switching logic.

Command.h

@@ -31,6 +31,7 @@ enum class CommandId {
 	VisualFontPickerToggle,
 	// Buffers
 	BufferSwitchStart, // begin buffer switch prompt
+	BufferNew, // create a new empty, unnamed buffer (C-k i)
 	BufferClose,
 	BufferNext,
 	BufferPrev,