Make TransactionIsolation an interface

Switched the enum to implementing the new interface. e.g., MS SQL has a snapshot isolation level, so best to not be strictly limited to an enum.

src/Link.php

@@ -9,5 +9,7 @@ interface Link extends Executor
      *
      * @param TransactionIsolation $isolation Transaction isolation level.
      */
-    public function beginTransaction(TransactionIsolation $isolation = TransactionIsolation::Committed): Transaction;
+    public function beginTransaction(
+        TransactionIsolation $isolation = TransactionIsolationLevel::Committed,
+    ): Transaction;
 }

src/TransactionIsolation.php

@@ -2,30 +2,15 @@
 
 namespace Amp\Sql;
 
-enum TransactionIsolation
+interface TransactionIsolation
 {
-    case Uncommitted;
-    case Committed;
-    case Repeatable;
-    case Serializable;
+    /**
+     * @return string Human-readable label for the transaction isolation level.
+     */
+    public function getLabel(): string;
 
-    public function getLabel(): string
-    {
-        return match ($this) {
-            self::Uncommitted => 'Uncommitted',
-            self::Committed => 'Committed',
-            self::Repeatable => 'Repeatable',
-            self::Serializable => 'Serializable',
-        };
-    }
-
-    public function toSql(): string
-    {
-        return match ($this) {
-            self::Uncommitted => 'READ UNCOMMITTED',
-            self::Committed => 'READ COMMITTED',
-            self::Repeatable => 'REPEATABLE READ',
-            self::Serializable => 'SERIALIZABLE',
-        };
-    }
+    /**
+     * @return string SQL to be inserted as the transaction isolation level.
+     */
+    public function toSql(): string;
 }

src/TransactionIsolationLevel.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Amp\Sql;
+
+enum TransactionIsolationLevel implements TransactionIsolation
+{
+    case Uncommitted;
+    case Committed;
+    case Repeatable;
+    case Serializable;
+
+    public function getLabel(): string
+    {
+        return match ($this) {
+            self::Uncommitted => 'Uncommitted',
+            self::Committed => 'Committed',
+            self::Repeatable => 'Repeatable',
+            self::Serializable => 'Serializable',
+        };
+    }
+
+    public function toSql(): string
+    {
+        return match ($this) {
+            self::Uncommitted => 'READ UNCOMMITTED',
+            self::Committed => 'READ COMMITTED',
+            self::Repeatable => 'REPEATABLE READ',
+            self::Serializable => 'SERIALIZABLE',
+        };
+    }
+}