[LSP] Support project context for roslyn LSP outside of VS

[dibarbet]

For VSCode, we need some mechanism of determining project context

1. A UI to allow user to choose project context (possibly as part of the extension)
2. A mechanism for the extension to piggy back on the LSP didOpen / intercept them to add the project context when sent to the server - possibly middleware - https://code.visualstudio.com/api/language-extensions/embedded-languages#request-forwarding-sample

[dibarbet]

cc @nohwnd

[CEbbinghaus]

Am I interpreting this Issue correctly as allowing for the LSP to work with projects only rather than requiring solutions?

[CyrusNajmabadi]

@dibarbet does this still need to be done? If so, please assign to a milestone.

[dibarbet]

moving to @JoeRobich as part of the project context switching in vscode