Use configured cadl-server path in VS when using a solution

[nguerrera]

Take 2 of #916. Did more cleanup (all documented below) and fixed bug where there are non-string values in the settings json file.

When a solution is open, there is no workspace, so we use the solution folder as the workspace folder and read the settings file ourselves.

Also fix an issue where there was not a good user-facing message when cadl-server was configured to a path that did not exist.

Finally, some general cleanup:

• Downgrade 16.x VS API packages to 16.0 for maximum compatibility
• Upgrade other packages to latest
• Run VS format command to format #if blocks not formatted by dotnet format
• Run VS remove-and-sort-usings
• Add inner exceptions to our exception types
• Convert VariableResolver to static class
• Avoid checking development mode in two places
• Make helper types internal
• Seal classes where possible
• Use Dictionary.Add instead of indexer to get exception on duplicates
• Use TryGetValue instead of catch KeyNotFoundException
• Use exception filter instead of if+rethrow
• Use string instead of string[] for process arguments
  • It was confusing me that we sometimes put more than one argument in a single array element.
• Use == or != null throughout instead of is and is not
  • I could go either way on this, but chose what we had most for consistency.
• Use multiple args to Path.Combine instead of one with forward slashes to get OS-idiomatic paths.

Fix #660

[azure-pipelines]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/prs/920/