Update README badges and improve alignment

Updated the `.NET Core` badge to reference the `v2_develop` branch.
Revised the `codecov` badge to remove branch-specific paths and
added a token parameter. Adjusted badge placement for better
alignment and included additional badges for `Downloads`,
`License`, and `Bugs`.

README.md

@@ -1,6 +1,6 @@
-![.NET Core](https://github.com/gui-cs/Terminal.Gui/workflows/.NET%20Core/badge.svg?branch=develop)
+![.NET Core](https://github.com/gui-cs/Terminal.Gui/workflows/.NET%20Core/badge.svg?branch=v2_develop)
 [![Version](https://img.shields.io/nuget/v/Terminal.Gui.svg)](https://www.nuget.org/packages/Terminal.Gui)
-[![codecov](https://codecov.io/gh/gui-cs/Terminal.Gui/branch/v2_develop/graph/badge.svg)](https://codecov.io/gh/gui-cs/Terminal.Gui)
+[![codecov](https://codecov.io/gh/gui-cs/Terminal.Gui/graph/badge.svg?token=1Ac9gyGtrj)](https://codecov.io/gh/gui-cs/Terminal.Gui)
 [![Downloads](https://img.shields.io/nuget/dt/Terminal.Gui)](https://www.nuget.org/packages/Terminal.Gui)
 [![License](https://img.shields.io/github/license/gui-cs/gui.cs.svg)](LICENSE)
 ![Bugs](https://img.shields.io/github/issues/gui-cs/gui.cs/bug)