Add a free-text "description" field to `serverInfo`

It would be nice to have a free-text (or perhaps markdown) field in serverInfo for the server to provide any server-specific information that it thinks the user might want to see.

Here's an example. The Haskell language server supports different features depending which version of the Haskell compiler it is built for (typically we start with partial support for a new compiler version and then add more over time). This means that users can be surprised that some feature is not available. At the moment, we have no easy way of conveying this to the user. The information is HLS-specific, so it's not clear that we could have a standard format for "server features". The simplest thing would just be to have a free text field so the server can put whatever it wants in there.

We can of course emit this information to the log, but it would be nice for users to have a standard place where static information about the server was recorded.

Start by locating the serverInfo definition in the language server protocol specification and review how existing server metadata fields are represented. Decide whether the new value is plain text or markdown, where it belongs, and how clients should expose it; done means the protocol shape and its intended client-facing semantics are clearly specified.