Made strings indexable

UpsilonTests/StringTests.cs

@@ -0,0 +1,51 @@
+using Upsilon.Evaluator;
+using Xunit;
+
+namespace UpsilonTests
+{
+    public class StringTests
+    {
+        [Fact]
+        public void BasicStringVariable()
+        {
+            const string input = @"
+string = ""test""
+return string
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            var evaluated = script.Evaluate<string>();
+            Assert.Empty(script.Diagnostics.Messages);
+            Assert.Equal("test", evaluated);
+        }
+
+        [Fact]
+        public void StringIndexable()
+        {
+            const string input = @"
+string = ""test""
+return string[3]
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            var evaluated = script.Evaluate<string>();
+            Assert.Empty(script.Diagnostics.Messages);
+            Assert.Equal("s", evaluated);
+        }
+
+        [Fact]
+        public void StringAddition()
+        {
+            const string input = @"
+string = ""test"" + ""123""
+return string
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            var evaluated = script.Evaluate<string>();
+            Assert.Empty(script.Diagnostics.Messages);
+            Assert.Equal("test123", evaluated);
+        }
+
+    }
+}