This is the last of the big changes to the js code

This Pr largely tightens up a lot of the code. 'v8' is no longer imported
outside of js. A number of helper functions have been moved to the js.Context.
For example, js.Function.getName used to call:

```zig
return js.valueToString(allocator, name, self.context.isolate, self.context.v8_context);
```

It now calls:

```zig
return self.context.valueToString(name, .{ .allocator = allocator });
```

Page.main_context has been renamed to `Page.js`. This, in combination with new
promise helpers, turns:

```zig
const resolver = page.main_context.createPromiseResolver();
try resolver.resolve({});
return resolver.promise();
```

into:

```zig
return page.js.resolvePromise({});
```

src/testing.zig

@@ -395,7 +395,7 @@ pub fn htmlRunner(file: []const u8) !void {
 
     page.arena = @import("root").tracking_allocator;
 
-    const js_context = page.main_context;
+    const js_context = page.js;
     var try_catch: js.TryCatch = undefined;
     try_catch.init(js_context);
     defer try_catch.deinit();
@@ -409,7 +409,7 @@ pub fn htmlRunner(file: []const u8) !void {
     page.session.browser.runMessageLoop();
 
     const needs_second_wait = try js_context.exec("testing._onPageWait.length > 0", "check_onPageWait");
-    if (needs_second_wait.value.toBool(page.main_context.isolate)) {
+    if (needs_second_wait.value.toBool(page.js.isolate)) {
         // sets the isSecondWait flag in testing.
         _ = js_context.exec("testing._isSecondWait = true", "set_second_wait_flag") catch {};
         _ = page.wait(2000);