refactor AST to separate them into their own files (Azure#53454)

Co-authored-by: Copilot <[email protected]>

Author: ArcturusZhang

sdk/provisioning/Azure.Provisioning/src/Expressions/AST.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+namespace Azure.Provisioning.Expressions;
+
+public class BicepProgram(params BicepStatement[] body)
+{
+    public BicepStatement[] Body { get; } = body;
+    public string? ModuleName { get; set; }
+    public override string ToString()
+    {
+        BicepWriter writer = new();
+        // if (ModuleName != null) { writer.Append("// module ").Append(ModuleName).AppendLine(); }
+        foreach (BicepStatement statement in Body)
+        {
+            writer = writer.Append(statement).AppendLine();
+        }
+        return writer.ToString();
+    }
+}

sdk/provisioning/Azure.Provisioning/src/Expressions/AST/BicepProgram.cs

@@ -0,0 +1,22 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+namespace Azure.Provisioning.Expressions;
+
+public class ArrayExpression(params BicepExpression[] values) : BicepExpression
+{
+    public BicepExpression[] Values { get; } = values;
+    internal override BicepWriter Write(BicepWriter writer)
+    {
+        if (Values.Length == 0)
+        {
+            return writer.Append("[]");
+        }
+        else
+        {
+            return writer.Append('[')
+                .Indent(w => w.AppendLine().AppendAll(Values, (w, v) => w.Append(v), w => w./*Append(',').*/AppendLine()))
+                .AppendLine().Append(']');
+        }
+    }
+}