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

src/nvim/log.h

@@ -17,10 +17,11 @@
 #endif
 
 
-#define DEBUG_LOG_LEVEL 0
-#define INFO_LOG_LEVEL 1
-#define WARN_LOG_LEVEL 2
-#define ERROR_LOG_LEVEL 3
+#define TRACE_LOG_LEVEL 0
+#define DEBUG_LOG_LEVEL 1
+#define INFO_LOG_LEVEL 2
+#define WARN_LOG_LEVEL 3
+#define ERROR_LOG_LEVEL 4
 
 #define DLOG(...)
 #define DLOGN(...)

src/nvim/lua/vim.lua

@@ -39,6 +39,16 @@ assert(vim)
 vim.inspect = package.loaded['vim.inspect']
 assert(vim.inspect)
 
+vim.log = {
+  levels = {
+    TRACE = 0;
+    DEBUG = 1;
+    INFO  = 2;
+    WARN  = 3;
+    ERROR = 4;
+  }
+}
+
 -- Internal-only until comments in #8107 are addressed.
 -- Returns:
 --    {errcode}, {output}
@@ -478,6 +488,24 @@ function vim.defer_fn(fn, timeout)
   return timer
 end
 
+
+--- Notification provider
+--- without a runtime, writes to :Messages
+--
+--@param msg Content of the notification to show to the user
+--@param log_level Optional log level
+--@param opts Dictionary with optional options (timeout, etc)
+--@param timeout Number of milliseconds to wait before calling `fn`
+function vim.notify(msg, log_level, _opts)
+
+  if log_level == vim.log.levels.ERROR then
+    vim.api.nvim_err_writeln(msg)
+  else
+    vim.api.nvim_echo(msg)
+  end
+end
+
+
 local on_keystroke_callbacks = {}
 
 --- Register a lua {fn} with an {id} to be run after every keystroke.