Add dotnet template for creating a new bonsai environment

[glopesdev]

This PR adds support for bootstrapping a new self-contained Bonsai environment using the dotnet templating system, PowerShell, and GitHub releases.

To install:

dotnet new -i Bonsai.Templates

Create a new environment folder:

dotnet new bonsaienv -o bonsai

If there is an existing Bonsai.config and/or NuGet.config in the folder, these will be used as part of the environment reconstruction process. In this case, it is enough to navigate to the folder where Bonsai.config is and simply run:

dotnet new bonsaienv