Add UndefinedOr(T) union

Some apis want a value or undefined. For these, we can't use an Optional
return type, null maps to JS null. Adds an Env.UndefinedOr(T) generic
union for such return types.

src/browser/html/DataSet.zig

@@ -17,6 +17,7 @@
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 const std = @import("std");
 const parser = @import("../netsurf.zig");
+const Env = @import("../env.zig").Env;
 const Page = @import("../page.zig").Page;
 
 const Allocator = std.mem.Allocator;
@@ -25,16 +26,12 @@ const DataSet = @This();
 
 element: *parser.Element,
 
-const GetResult = union(enum) {
-    value: []const u8,
-    undefined: void,
-};
-pub fn named_get(self: *const DataSet, name: []const u8, _: *bool, page: *Page) !GetResult {
+pub fn named_get(self: *const DataSet, name: []const u8, _: *bool, page: *Page) !Env.UndefinedOr([]const u8) {
     const normalized_name = try normalize(page.call_arena, name);
     if (try parser.elementGetAttribute(self.element, normalized_name)) |value| {
         return .{ .value = value };
     }
-    return .{ .undefined = {} };
+    return .undefined;
 }
 
 pub fn named_set(self: *DataSet, name: []const u8, value: []const u8, _: *bool, page: *Page) !void {

src/browser/html/error_event.zig

@@ -72,15 +72,11 @@ pub const ErrorEvent = struct {
         return self.colno;
     }
 
-    const ErrorValue = union(enum) {
-        obj: Env.JsObject,
-        undefined: void,
-    };
-    pub fn get_error(self: *const ErrorEvent) ErrorValue {
+    pub fn get_error(self: *const ErrorEvent) Env.UndefinedOr(Env.JsObject) {
         if (self.@"error") |e| {
-            return .{ .obj = e };
+            return .{ .value = e };
         }
-        return .{ .undefined = {} };
+        return .undefined;
     }
 };
 

src/runtime/js.zig

@@ -1858,6 +1858,13 @@ pub fn Env(comptime State: type, comptime WebApis: type) type {
             }
         };
 
+        pub fn UndefinedOr(comptime T: type) type {
+            return union(enum) {
+                value: T,
+                undefined: void,
+            };
+        }
+
         fn compileModule(isolate: v8.Isolate, src: []const u8, name: []const u8) !v8.Module {
             // compile
             const script_name = v8.String.initUtf8(isolate, name);