Handle if else if (else) statements

UpsilonTests/IfTests.cs

@@ -34,5 +34,26 @@ end";
             var actual = (long)(NumberLong)script.Evaluate();
             Assert.Equal(expected, actual);
         }
+
+        [Theory]
+        [InlineData("true", "false", 3, 8, 5, 3)]
+        [InlineData("false", "true", 3, 8, 5, 8)]
+        public void BasicIfElseIfElseTests(string condition1, string condition2, int in1, int in2, int in3, long expected)
+        {
+            var input =
+                $@"
+if {condition1} then
+    val = {in1}
+elseif {condition2} then
+    val = {in2}
+else
+    val = {in3}
+end";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            var actual = (long)(NumberLong)script.Evaluate();
+            Assert.Equal(expected, actual);
+        }
+
     }
 }