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.

KKeymap.cc

@@ -42,6 +42,9 @@ KLookupKCommand(const int ascii_key, const bool ctrl, CommandId &out) -> bool
 	case 'a':
 		out = CommandId::MarkAllAndJumpEnd;
 		return true;
+	case 'i':
+		out = CommandId::BufferNew; // C-k i new empty buffer
+		return true;
 	case 'k':
 		out = CommandId::CenterOnCursor; // C-k k center current line
 		return true;