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/browser/js/Function.zig

@@ -22,7 +22,7 @@ pub const Result = struct {
 
 pub fn getName(self: *const Function, allocator: Allocator) ![]const u8 {
     const name = self.func.castToFunction().getName();
-    return js.valueToString(allocator, name, self.context.isolate, self.context.v8_context);
+    return self.context.valueToString(name, .{ .allocator = allocator });
 }
 
 pub fn setName(self: *const Function, name: []const u8) void {