feat: allow configuration through workspace/didChangeConfiguration

[willruggiano]

What kind of change does this PR introduce?

It enables passing workspace settings via the workspace/didChangeConfiguration notification. This in turn enables the client to specify settings dynamically, rather than being limited to configuration files. This is a solution to #302. See example usage (with lspconfig) here: willruggiano/neovim.drv@9aa06ad.

What is the current behavior?

There is none. The payload of this handler is currently ignored.

What is the new behavior?

The configuration received by the handler is merged with the fs configuration.

[willruggiano]

This is literally my first time writing Rust, ever. lmao.

Furthermore, I started this in draft because I'm not sure what you want with regards to testing and such. Is it safe to change workspace settings at runtime? No idea. Is my code nasty? Certainly.

Anyways, let me know how/if you'd like to proceed with this. I thought it would be pretty straightforward after thinking it through while reading #302 and, I don't know, kinda felt the random urge to finally give Rust a try :)