Suggest to rebuild project in case of code changes

Sometimes, users change the code outside their XAML, and expect the previewer to take these changes into account.

In those cases, DLL rebuild is required, and the plugin doesn't help people to determine that.

I think we could do better.

1. We can just watch over the `.cs` file changes and suggest project rebuild if needed.
2. We can also instrument the run using dotCover to better understand _what exact lines of code_ are used in the project and what aren't, and only suggest rebuild if the changes actually affect the currently shown preview.

The issue names .cs files, XAML previews, DLL rebuilds, and optional dotCover instrumentation, but no repository files or tests. Start by locating how the previewer detects project changes and triggers runs, then clarify whether the scope is rebuild prompting or coverage-based filtering. Done means the preview clearly prompts for a required rebuild without unnecessary prompts.

Written by the indexing model from the issue text.