[O# Parity] Support loading non-solution based projects/files

[dibarbet]

Currently the C# extension's project loading capabilities only supports loading full solution files. O# supports loading csproj files.

CSX support is tracked here - #6411

[alexrp]

I see this is in "feature backlog", so I just want to ask what that means in terms of prioritization?

I use Traversal for basically every project I work on these days, which works great under OmniSharp since it can load loose project files, and in VS with Open Folder too. It would be unfortunate to have to take a step backwards and maintain solution files in order to use the new Roslyn-based LSP host. (FWIW, judging by the reactions on microsoft/vscode-dotnettools#121, others are in the same boat.)