Keep the scroll target types and wheelScroll file-private

ScrollTarget and ScrollTargets are only named inside Element.zig; callers
switch on the result or call scrollBy through it. wheelScroll has one
caller, wheel, in the same file.

src/browser/frame/user_input.zig

@@ -325,7 +325,7 @@ pub fn wheel(frame: *Frame, target: *Element, x: f64, y: f64, delta_x: f64, delt
 /// Each axis scrolls the nearest ancestor-or-self scroll container along it,
 /// else the viewport. Relative deltas may land on different scrollers per
 /// axis, unlike an absolute position.
-pub fn wheelScroll(target: *Element, delta_x: i32, delta_y: i32, frame: *Frame) !void {
+fn wheelScroll(target: *Element, delta_x: i32, delta_y: i32, frame: *Frame) !void {
     const targets = target.scrollContainers(.{ .x = delta_x != 0, .y = delta_y != 0 }, frame);
     // A zero delta resolves to .viewport and scrolls it by nothing.
     try targets.x.scrollBy(delta_x, 0, frame);

src/browser/webapi/Element.zig

@@ -1611,7 +1611,7 @@ pub const ScrollAxes = struct { x: bool = false, y: bool = false };
 
 /// What a scroll along one axis lands on. html and body scroll the viewport,
 /// and so does a detached element.
-pub const ScrollTarget = union(enum) {
+const ScrollTarget = union(enum) {
     viewport,
     container: *Element,
 
@@ -1624,7 +1624,7 @@ pub const ScrollTarget = union(enum) {
     }
 };
 
-pub const ScrollTargets = struct {
+const ScrollTargets = struct {
     x: ScrollTarget = .viewport,
     y: ScrollTarget = .viewport,
 };