Implement configurable log level for language server's built-in logger.

Resolves #69.

Author: tintoy

src/Server/LanguageServer.cs

@@ -90,6 +90,11 @@ internal LanguageServer(
         public InitializeParams Client { get; private set; }
         public InitializeResult Server { get; private set; }
 
+        /// <summary>
+        ///     The minimum level for the server's default logger.
+        /// </summary>
+        public LogLevel MinimumLogLevel { get; set; } = LogLevel.Information;
+
         public IDisposable AddHandler(string method, IJsonRpcHandler handler)
         {
             var handlerDisposable = _collection.Add(method, handler);

src/Server/LanguageServerLogger.cs

@@ -20,14 +20,13 @@ public IDisposable BeginScope<TState>(TState state)
             return new ImmutableDisposable();
         }
 
-        public bool IsEnabled(LogLevel logLevel)
-        {
-            // TODO: setup as configuration somehwhere (from trace perhaps?)
-            return true;
-        }
+        public bool IsEnabled(LogLevel logLevel) => logLevel >= _languageServer.MinimumLogLevel;
 
         public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
         {
+            if (logLevel < _languageServer.MinimumLogLevel)
+                return;
+
             if (TryGetMessageType(logLevel, out var messageType))
             {
                 _languageServer.Log(new LogMessageParams()