Generalize `configSection` to a list

As far as I understand, the current version supports either empty string here (give me all configuration), or a name, in which case you'd get one section.

I run into a situation where it would be convenient to request more than one section, even though this is probably not all that
common. Concretely, I was working on a plug-in that processes some C files, and I wanted to also be able to access the configuration of another C plugin (I know that's not going to work if the other plugin is not installed, but for my use case it is reasonable to assume that it is).

My request would be to either change the type to a list of `Text`, or if we want to be somewhat backward compatible, have a way to specify multiple sections in the string (e.g., `;` separated).

Start by tracing the configSection definition and its consumers in the Haskell LSP library. Compare changing it to a list with a backward-compatible separator approach, and consider behavior when a requested plugin is not installed; done means multiple configuration sections can be requested without breaking existing single-section or empty-string behavior.

Written by the indexing model from the issue text.