use V8 json parser with xhr/fetch webAPIs

The pure zig JSON parser didn't generate the same type of values than JS
JSON.parse command.
Using directly V8's JSON parser gives the assurance to have the right
JS types.
Moreover, it avoid data transformations between Zig and V8.

src/browser/fetch/Request.zig

@@ -254,17 +254,12 @@ pub fn _json(self: *Response, page: *Page) !js.Promise {
     self.body_used = true;
 
     if (self.body) |body| {
-        const p = std.json.parseFromSliceLeaky(
+        const value = js.Value.fromJson(page.js, body) catch |e| {
-            std.json.Value,
-            page.call_arena,
-            body,
-            .{},
-        ) catch |e| {
             log.info(.browser, "invalid json", .{ .err = e, .source = "Request" });
             return error.SyntaxError;
         };
 
-        return page.js.resolvePromise(p);
+        return page.js.resolvePromise(value);
     }
     return page.js.resolvePromise(null);
 }

src/browser/fetch/Response.zig

@@ -179,17 +179,12 @@ pub fn _json(self: *Response, page: *Page) !js.Promise {
 
     if (self.body) |body| {
         self.body_used = true;
-        const p = std.json.parseFromSliceLeaky(
+        const value = js.Value.fromJson(page.js, body) catch |e| {
-            std.json.Value,
-            page.call_arena,
-            body,
-            .{},
-        ) catch |e| {
             log.info(.browser, "invalid json", .{ .err = e, .source = "Response" });
             return error.SyntaxError;
         };
 
-        return page.js.resolvePromise(p);
+        return page.js.resolvePromise(value);
     }
     return page.js.resolvePromise(null);
 }

src/browser/xhr/xhr.zig

@@ -31,6 +31,7 @@ const Mime = @import("../mime.zig").Mime;
 const parser = @import("../netsurf.zig");
 const Page = @import("../page.zig").Page;
 const Http = @import("../../http/Http.zig");
+const js = @import("../js/js.zig");
 
 // XHR interfaces
 // https://xhr.spec.whatwg.org/#interface-xmlhttprequest
@@ -128,21 +129,19 @@ pub const XMLHttpRequest = struct {
         JSON,
     };
 
-    const JSONValue = std.json.Value;
-
     const Response = union(ResponseType) {
         Empty: void,
         Text: []const u8,
         ArrayBuffer: void,
         Blob: void,
         Document: *parser.Document,
-        JSON: JSONValue,
+        JSON: js.Value,
     };
 
     const ResponseObj = union(enum) {
         Document: *parser.Document,
         Failure: void,
-        JSON: JSONValue,
+        JSON: js.Value,
 
         fn deinit(self: ResponseObj) void {
             switch (self) {
@@ -605,7 +604,7 @@ pub const XMLHttpRequest = struct {
     }
 
     // https://xhr.spec.whatwg.org/#the-response-attribute
-    pub fn get_response(self: *XMLHttpRequest) !?Response {
+    pub fn get_response(self: *XMLHttpRequest, page: *Page) !?Response {
         if (self.response_type == .Empty or self.response_type == .Text) {
             if (self.state == .loading or self.state == .done) {
                 return .{ .Text = try self.get_responseText() };
@@ -652,7 +651,7 @@ pub const XMLHttpRequest = struct {
             // TODO Let jsonObject be the result of running parse JSON from bytes
             // on this’s received bytes. If that threw an exception, then return
             // null.
-            self.setResponseObjJSON();
+            self.setResponseObjJSON(page);
         }
 
         if (self.response_obj) |obj| {
@@ -691,22 +690,18 @@ pub const XMLHttpRequest = struct {
         };
     }
 
-    // setResponseObjJSON parses the received bytes as a std.json.Value.
+    // setResponseObjJSON parses the received bytes as a js.Value.
-    fn setResponseObjJSON(self: *XMLHttpRequest) void {
+    fn setResponseObjJSON(self: *XMLHttpRequest, page: *Page) void {
-        // TODO should we use parseFromSliceLeaky if we expect the allocator is
+        const value = js.Value.fromJson(
-        // already an arena?
+            page.js,
-        const p = std.json.parseFromSliceLeaky(
-            JSONValue,
-            self.arena,
             self.response_bytes.items,
-            .{},
         ) catch |e| {
             log.warn(.http, "invalid json", .{ .err = e, .url = self.url, .source = "xhr" });
             self.response_obj = .{ .Failure = {} };
             return;
         };
 
-        self.response_obj = .{ .JSON = p };
+        self.response_obj = .{ .JSON = value };
     }
 
     pub fn get_responseText(self: *XMLHttpRequest) ![]const u8 {