update engine code assignment to use static declaration

Author: ceastwood

QueryBuilder/Compilers/FirebirdCompiler.cs

@@ -10,7 +10,7 @@ public FirebirdCompiler()
         {
         }
 
-        public override string EngineCode { get; } = "firebird";
+        public override string EngineCode { get; } = EngineCodes.Firebird;
 
         protected override SqlResult CompileInsertQuery(Query query)
         {

QueryBuilder/Compilers/MySqlCompiler.cs

@@ -8,7 +8,7 @@ public MySqlCompiler()
             LastId = "SELECT last_insert_id() as Id";
         }
 
-        public override string EngineCode { get; } = "mysql";
+        public override string EngineCode { get; } = EngineCodes.MySql;
 
         public override string CompileLimit(SqlResult ctx)
         {

QueryBuilder/Compilers/OracleCompiler.cs

@@ -1,6 +1,5 @@
 using System.Diagnostics;
 using System.Linq;
-using SqlKata.Compilers.Extensions;
 
 namespace SqlKata.Compilers
 {
@@ -13,7 +12,7 @@ public OracleCompiler()
             parameterPlaceholderPrefix = ":p";
         }
 
-        public override string EngineCode { get; } = OracleCompilerExtensions.ENGINE_CODE;
+        public override string EngineCode { get; } = EngineCodes.Oracle;
         public bool UseLegacyPagination { get; set; } = false;
 
         protected override SqlResult CompileSelectQuery(Query query)

QueryBuilder/Compilers/PostgresCompiler.cs

@@ -7,7 +7,7 @@ public PostgresCompiler()
             LastId = "SELECT lastval()";
         }
 
-        public override string EngineCode { get; } = "postgres";
+        public override string EngineCode { get; } = EngineCodes.PostgreSql;
 
         protected override string CompileBasicDateCondition(SqlResult ctx, BasicDateCondition condition)
         {

QueryBuilder/Compilers/SqlServerCompiler.cs

@@ -3,14 +3,13 @@ namespace SqlKata.Compilers
     public class SqlServerCompiler : Compiler
     {
         public SqlServerCompiler()
-
         {
             OpeningIdentifier = "[";
             ClosingIdentifier = "]";
             LastId = "SELECT scope_identity() as Id";
         }
 
-        public override string EngineCode { get; } = "sqlsrv";
+        public override string EngineCode { get; } = EngineCodes.SqlServer;
         public bool UseLegacyPagination { get; set; } = true;
 
         protected override SqlResult CompileSelectQuery(Query query)

QueryBuilder/Compilers/SqliteCompiler.cs

@@ -6,7 +6,7 @@ namespace SqlKata.Compilers
 {
     public class SqliteCompiler : Compiler
     {
-        public override string EngineCode => "sqlite";
+        public override string EngineCode { get; } = EngineCodes.Sqlite;
         protected override string parameterPlaceholder { get; set; } = "?";
         protected override string parameterPlaceholderPrefix { get; set; } = "@p";
         protected override string OpeningIdentifier { get; set; } = "\"";