Re-organize contexts to divide by shell

[ryanvolum]

We should add directories in the context folder for each shell, and probably an additional folder for shell-agnostic contexts (e.g. git, kubernetes, azure cli, etc.).

I expect folks may want to contribute their own contexts, and having a flat list could get overwhelming. Also, reducing keystroks for // load context powershell-conversational-context to => // load context conversational would be nice. In that example, knowing that we're using PowerShell should trigger where we load the context from.