Support exit notification

[etherbob]

Per a report from another user who tested out building and running fuzzy_rubby_server standalone:

I could manage to build and install the LSP by: git clone the project cd to the project directory build the executable : cargo build --release --target=aarch64-apple-darwin symlink the .../fuzzy_ruby_server/bin/fuzzy_darwin-arm64 binary into the standard bin/. directory configure BBEdit to use this binary for the ruby language The LSP seems to be working as expected and logged in the LanguageServerProtocol-Ruby.txt file. But there is a caveat: BBEdit will freeze when quitting, probably due to the LSP not behaving as expected when asked to shutdown. I suspect the LSP does not respond to the Exit Notification. {"jsonrpc":"2.0","id":2,"method":"shutdown"}Content-Length: 38

{"jsonrpc":"2.0","result":null,"id":2}Content-Length: 33

{"jsonrpc":"2.0","method":"exit"} ... If this is indeed the cause, it should be reported to the developer.

With the disclaimer that I've not done anything in rust before, but I didn't see anything in the server to handle "exit" requests. I also couldn't tell for certain if TowerLSP is supposed to handle exit requests without any additional implementation.