Addressed code review comments.

- Changed the RazorPage writing scopes to replace the entire ViewContext.Writer to ensure that we capture content from Html.BeginForm etc.
- Added a test to cover the ViewContext.Writer redirection scenario.
Add CreateTagHelper to RazorPage.

- The CreateTagHelper method is responsible for creating and activating TagHelpers.
- Added the support for requesting the ViewContext.
- Added tests to validate the tag helper creation mechanism.

#1104