Enable nullable reference types across the whole repo

[bekir-ozturk]

We have started using #nullable enable on the files we edit to help prevent null reference exceptions. Let's make sure that this effort reaches the completion state by enabling it repo-wide.

Once we are at "mostly done" state:

• Take the initiative to enable nullables on all the remaining files (hopefully, a small set of files).
• Remove #nullable enable from cs files and enable the checks on project level (csproj files).

Done:

• [x] dotnet-new3
• [x] Cli
• [x] IDE
• [x] Tasks
• [x] TemplateLocalizer and TemplateLocalizer.Core
• [x] TemplateSearch.Common
• [x] TemplateDiscovery
• [ ] Abstractions - high
• [ ] Edge - high
• [ ] Utils - high

Not done:

• [ ] Core and Core.Contracts
• [ ] Orchestration.RunnableProjects

Low prio:

• ScraperOutputComparison - not used
• tests