kill `dotnet watch` if nvim is exited

At the moment properly managing the pid state is borked. [see dotnet issue #20152](https://github.com/dotnet/aspnetcore/issues/20152).

As a result, `dotnvim` will be tackling this issue in neovim by adding a hook when neovim closes to send `dotnvim.kill_dotnet_watch` when neovim ends.