feat: adds vim.notify

Adds function to notify the user like this:
`:lua vim.notify("hello user")`
embeds log levels vim.log.levels.

you can then reassign vim.notify to for instance
```
function notify_external(msg, log_level, opts)
	vim.fn.jobstart({"notify-send", msg })
end
```

runtime/lua/vim/lsp/log.lua

@@ -10,13 +10,7 @@ local log = {}
 -- Can be used to lookup the number from the name or the name from the number.
 -- Levels by name: 'trace', 'debug', 'info', 'warn', 'error'
 -- Level numbers begin with 'trace' at 0
-log.levels = {
-  TRACE = 0;
-  DEBUG = 1;
-  INFO  = 2;
-  WARN  = 3;
-  ERROR = 4;
-}
+log.levels = vim.log.levels
 
 -- Default log level is warn.
 local current_log_level = log.levels.WARN