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/CompoundSqlToolsSettingsValues.cs

@@ -0,0 +1,102 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.SqlTools.Utility;
+using Newtonsoft.Json;
+
+namespace Microsoft.SqlTools.ServiceLayer.SqlContext
+{
+    
+    /// <summary>
+    /// Handles backwards compatibility of settings by checking for settings in a priority list. If a settings
+    /// group such as Intellisense is defined on a serialized setting it's used in the order of mssql, then sql, then
+    /// falls back to a default value.
+    /// </summary>
+    public class CompoundToolsSettingsValues: ISqlToolsSettingsValues
+    {
+        private List<ISqlToolsSettingsValues> priorityList = new List<ISqlToolsSettingsValues>();
+        private SqlToolsSettingsValues defaultValues;
+        public CompoundToolsSettingsValues(ISqlToolsSettingsValues mssql, ISqlToolsSettingsValues all)
+        {
+            Validate.IsNotNull(nameof(mssql), mssql);
+            Validate.IsNotNull(nameof(all), all);
+            priorityList.Add(mssql);
+            priorityList.Add(all);
+            // Always add in a fallback which has default values to be used.
+            defaultValues = new SqlToolsSettingsValues(createDefaults: true);
+            priorityList.Add(defaultValues);
+        }
+
+        private T GetSettingOrDefault<T>(Func<ISqlToolsSettingsValues, T> lookup)
+            where T : new()
+        {
+            T value = priorityList.Select( (settings) => lookup(settings)).Where(val => val != null).FirstOrDefault();
+            return value != null ? value : new T();
+        }
+
+        /// <summary>
+        /// Gets or sets the detailed IntelliSense settings
+        /// </summary>
+        public IntelliSenseSettings IntelliSense
+        { 
+            get
+            {
+                return GetSettingOrDefault((settings) => settings.IntelliSense);
+            } 
+            set
+            {
+                priorityList[0].IntelliSense = value;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the query execution settings
+        /// </summary>
+        public QueryExecutionSettings QueryExecutionSettings
+        { 
+            get
+            {
+                return GetSettingOrDefault((settings) => settings.QueryExecutionSettings);
+            } 
+            set
+            {
+                priorityList[0].QueryExecutionSettings = value;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the formatter settings
+        /// </summary>
+        public FormatterSettings Format
+        { 
+            get
+            {
+                return GetSettingOrDefault((settings) => settings.Format);
+            } 
+            set
+            {
+                priorityList[0].Format = value;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the object explorer settings
+        /// </summary>
+        public ObjectExplorerSettings ObjectExplorer
+        { 
+            get
+            {
+                return GetSettingOrDefault((settings) => settings.ObjectExplorer);
+            } 
+            set
+            {
+                priorityList[0].ObjectExplorer = value;
+            }
+        }
+    }
+}