Add option to navigate proper dlls with IL Viewer

Currently IL Viewer inspects dlls builds manually in Rider but they don't have code generated for ex. by Mono.Cecil.

If it can't be default then there should be an option to point csproj OutputPath to `Library\ScriptAssemblies` as was reverted in this commit https://github.com/JetBrains/resharper-unity/commit/f5a6fea6fb7be4ea2df5f4c120027ead0a1d0eb2 .

Changed OutputPath to Library\ScriptAssemblies locally in your package and everything seems fine. Why it can't be like that by default? Build button should also be disabled in that case as it's pointless in Unity that doesn't use MSBuild but their own solution.

Start by reviewing the IL Viewer handling of DLLs and the reverted commit f5a6fea6fb7be4ea2df5f4c120027ead0a1d0eb2. Trace how the csproj OutputPath relates to Library\ScriptAssemblies and define completion as supporting the requested option or default while disabling the Build button when Unity does not use MSBuild.

Written by the indexing model from the issue text.