feat(app): event line provides entire line contents as a single event; add(event): .cancel event

Introduce `.cancel` event that is fired when the user sends EOF
in *non raw mode* renderings. The event `.line` now sends the
entire line (even if larger than the internal buffer) but requires
now an `Allocator`.

examples/elements/button.zig

@@ -82,7 +82,7 @@ pub fn main() !void {
 
     const allocator = gpa.allocator();
 
-    var app: App = .init(.{}, .{});
+    var app: App = .init(allocator, .{}, .{});
     var renderer = zterm.Renderer.Buffered.init(allocator);
     defer renderer.deinit();