EnC: Emit addition instead of in-place update for edits in reloadable types

[tmat]

• Define attribute that marks a reloadable type
  Consider Reloadable Attribute for Hot Reload  runtime#54633
• Recognize the attribute in the EnC analyzer and instruct compiler to emit "addition" instead of "update" on type level.
  The new type name should be unique, synthesized and unutterable.
• Determine how to apply these changes. Should be in a separate delta or together with in-place updates?
  Separate deltas could be sent to a different interface than the debugger interface and applied without the debugger being attached. This would however require the IDE to watch for changes not only when the debugging session is in progress but also in design-time mode when the app is running. This would be a significant amount of work in the IDE. Seems like we should start with the debugger applying both changes and separate later if we decide to support applying when running scenario.
• Debugger needs to trigger an event in the debuggee - via func-eval.
  The event has well-known full name with specific signature. It may be declared in any assembly and have any accessibility. Design with BCL.
• Updated types needs to be passed to runtime handler
  dotnet-watch: Update agents to pass the list of changed types to MetadataUpdateHandlers aspnetcore#33792
  WatchHotReloadService.EmitSolutionUpdateAsync should include the list of changed types #54336
• Renaming: Allow renaming reloadable types #54886

[tmat]

Implemented by #54960