Improve plugin registration

[rezanid]

Based on feedback reported by developers, the following improvements have been identified:

1. Currently when a plugin type is missing and plugin registration is called at project level, the confirmation that non-existing plugins will be removed, is not displayed. This has been done because removing and adding any type in assembly is considered natural and the mere existence of plugins types in assembly is the source of truth.

Still, it can be beneficial to display a prompt with a clear list of plugin types that will be removed. This behavior can be turned off via Options.

2. Plugin file detection -- which is used when displaying "Register Plugin(s)" command is based on plugin code generation being active for a file. We can switch to detection based on syntax tree, with the existing logic as a short-circuit. This will allow developers to register individual plugins even if they have not enabled plugin code generation for them.

Reference:

• Decouple attribute-based plugin/step/image registration from code generation (.g.cs) #170
• Incremental adoption: register a single plugin without defining attributes for the whole assembly? #165