Test LSP objects

[fm-117]

Our implementation of LSP is quite old now and more importantly is embedded instead of relying on a well-known externally-provided package.

Migrating to Microsoft.VisualStudio.LanguageServer.Protocol NuGet package is the target however this will be difficult as we have accumulated a lot of gaps over time.

The goal of this issue is to create tests covering the form of protocol objects. This will allow:

• to identify discrepancies between our private client definitions and server definitions
• to identify discrepancies between multiple version of the protocol objects (when migrating to VS package)

In a nutshell the protocol test should:

• automatically discover the messages known by this server
• verify that each message is actually covered by a test
• test every message using predefined JSON test files