adjust LSP storage scheme to be usable by multiple instances simultaneously

[GitMensch]

the only remaining option is to store the cache in a shared location (like in the project itself) and access it via multiple LSP servers. At the moment that's not supported (the cache itself is a single file that's juste blindly read on LSP server startup and written on shutdown), but I think moving towards a different storage scheme for the cache would not be too complicated.

Originally posted by @nberth in #343 (comment)

So this issue is about a possible adjustment. And yes, for bitlang.cobol (an older version where this was possible) we use the setup of a cache directory below the project (outside of the directories from the workspace as there's no use in indexing it). Note: that extension has an explicit command to index the whole workspace (and another to check for necessary updates on startup) which allows fast LSP access later (the cache entries are updated on file writes and are then available for anyone directly).