Support "SQL" settings in addition to MSSQL (#398)

* Support "SQL" settings in addition to MSSQL
- Handles having 2 separate configuration definitions and merging / treating them as 1 throughout the app
- If a settings group such as Intellisense is defined on mssql, it will override any generic SQL properties
- Retains backwards compatibility with existing settings.

src/Microsoft.SqlTools.ServiceLayer/SqlContext/ISqlToolsSettingsValues.cs

@@ -0,0 +1,33 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+namespace Microsoft.SqlTools.ServiceLayer.SqlContext
+{
+    /// <summary>
+    /// Defines the common settings used by the tools service
+    /// </summary>
+    public interface ISqlToolsSettingsValues
+    {
+        /// <summary>
+        /// Intellisense specific settings
+        /// </summary>
+        IntelliSenseSettings IntelliSense { get; set; }
+
+        /// <summary>
+        /// Query execution specific settings
+        /// </summary>
+        QueryExecutionSettings QueryExecutionSettings { get; set; }
+
+        /// <summary>
+        /// Formatter settings
+        /// </summary>
+        FormatterSettings Format { get; set; }
+
+        /// <summary>
+        /// Object Explorer specific settings
+        /// </summary>
+        ObjectExplorerSettings ObjectExplorer { get; set; }
+    }
+}