Avalonia.Ide.CompletionEngine: nullable, trimming, STJ, code format

[maxkatz6]

Updated Avalonia.Ide.CompletionEngine projects to be up to date with common style rules and nullability props same as main repository. Specifically:

• Nullable enabled
• net6 is an additional target (not really used, but can be used if we upload this package to the nuget)
• Records are used where makes sense
• Multiple potential issue are fixed with nullables
• At least one bug with IImage using code completion from IBitmap was fixed (copy paste error)
• System.Text.Json instead of Newtonsoft. Not really a beneficial change, but there is no reason to bring old dependency here.
• Copy paste editor config from main repo and formatted code
• File scope namespaces (technically, it makes git blame harder to use, but in fact all of these files were copy pasted from another repository, so there is no much difference).