fix: rust: disable unresolved-proc-macro

Since we disabled proc macro expansion, disable the related info
level lint:

https://users.rust-lang.org/t/how-to-disable-rust-analyzer-proc-macro-warnings-in-neovim/53150/3

languages.toml

@@ -11,6 +11,7 @@ indent = { tab-width = 4, unit = "    " }
 [language.config]
 cargo = { loadOutDirsFromCheck = true }
 procMacro = { enable = false }
+diagnostics = { disabled = ["unresolved-proc-macro"] }
 
 [[language]]
 name = "toml"