apprt: make gotoTab handle all tab movements

src/apprt/structs.zig

@@ -62,6 +62,16 @@ pub const DesktopNotification = struct {
     body: []const u8,
 };
 
+/// The tab to jump to. This is non-exhaustive so that integer values represent
+/// the index (zero-based) of the tab to jump to. Negative values are special
+/// values.
+pub const GotoTab = enum(c_int) {
+    previous = -1,
+    next = -2,
+    last = -3,
+    _,
+};
+
 // This is made extern (c_int) to make interop easier with our embedded
 // runtime. The small size cost doesn't make a difference in our union.
 pub const SplitDirection = enum(c_int) {