edit/createRow Default Values (#266)

Returns strings for the default value of a column when a new row is created. The values that could come back:
* `null` when there isn't a default for the column
* a string when there is a default for the column
* a placeholder (currently <TBD>) when the column cannot be updated

* Renaming EditTableMetadata to reflect its SMO source

* Implementation of returning default values

* Unit test for default values

* Reworking column defaults using default constraints

* Adding unit test for new sql script unwrapper

* Disabling flaky test

* Fixing oddities in tests, removing personal tests

* Fixing broken unit test

src/Microsoft.SqlTools.ServiceLayer/Utility/SqlScriptFormatter.cs

@@ -9,6 +9,7 @@ using System.Data.Common;
 using System.Globalization;
 using System.Linq;
 using System.Text;
+using System.Text.RegularExpressions;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
 using Microsoft.SqlTools.Utility;
 
@@ -64,6 +65,19 @@ namespace Microsoft.SqlTools.ServiceLayer.Utility
                 // sysname           - it doesn't appear possible to insert a sysname column
            };
 
+        private static readonly Type[] NumericTypes = 
+        {
+            typeof(byte),
+            typeof(short),
+            typeof(int),
+            typeof(long),
+            typeof(decimal),
+            typeof(float),
+            typeof(double)
+        };
+
+        private static Regex StringRegex = new Regex("^N?'(.*)'$", RegexOptions.Compiled);
+
         #endregion
 
         /// <summary>
@@ -141,6 +155,27 @@ namespace Microsoft.SqlTools.ServiceLayer.Utility
             return string.Join(".", escapedParts);
         }
 
+        /// <summary>
+        /// Converts a value from a script into a plain version by unwrapping literal wrappers
+        /// and unescaping characters.
+        /// </summary>
+        /// <param name="literal">The value to unwrap</param>
+        /// <returns>The unwrapped/unescaped literal</returns>
+        public static string UnwrapLiteral(string literal)
+        {
+            // Always remove parens
+            literal = literal.Trim('(', ')');
+
+            // Attempt to unwrap inverted commas around a string
+            Match match = StringRegex.Match(literal);
+            if (match.Success)
+            {
+                // Like: N'stuff' or 'stuff'
+                return UnEscapeString(match.Groups[1].Value, '\'');
+            }
+            return literal;
+        }
+
         #region Private Helpers
 
         private static string SimpleFormatter(object value)
@@ -258,6 +293,14 @@ namespace Microsoft.SqlTools.ServiceLayer.Utility
             return sb.ToString();
         }
 
+        private static string UnEscapeString(string value, char escapeCharacter)
+        {
+            Validate.IsNotNull(nameof(value), value);
+
+            // Replace 2x of the escape character with 1x of the escape character
+            return value.Replace(new string(escapeCharacter, 2), escapeCharacter.ToString());
+        }
+
         #endregion
     }
 }