Update templates

[qqrz997]

Replaces the current templates with 3 new ones:

• A bare minimum setup which only contains a Plugin class;
• A BSML setup which contains a simple but functional example menu;
• And a SiraUtil setup which uses the prior BSML menu and an additional singleton manager.

All code is designed with nullable reference types and an option is provided to disable nullable.

The projects use the latest beta of BSMT and XML-based metadata.

All templates contain many helpful hints in the form of comments which can be disabled on project creation.

All templates should be immediately compilable after generating the .csproj.user, and they do something in game to confirm that they are working (in the case of the bare plugin that would just be logging a message).