LSP support

[pieter-bos]

I think the language server protocol is a good umbrella under which we can assimilate the frontends of VerCors, which are:

• the command line utility
• the VSCode IDE
• the online runner

Some considerations:

• Progress in terms of percentage is natively supported
• Verification errors can be diagnostics (investigate whether pull or push)
• I think most warnings relate to an Origin, so those can be refactored to diagnostics as well
• Origin/Readable should likely be more directly tied to a path/uri
• I'm hoping custom data is commonly accepted, so prefix a JSON key with "vercors" or so.
• We can push the options related to verification (not all command line options - which?) to the client, hopefully making the IDE mostly backwards compatible

To be clear: I really intend to implement the command line utility on top of LSP, to encourage implementing features in all frontends in parallel. Preferably VerCors usually runs as a daemon the command line utility connects to, but let's start with just separating the interface.