Implement defining functions

UpsilonTests/FunctionTests.cs

@@ -0,0 +1,30 @@
+using Upsilon.BaseTypes;
+using Upsilon.BaseTypes.Number;
+using Upsilon.Evaluator;
+using Xunit;
+
+namespace UpsilonTests
+{
+    public class FunctionTests
+    {
+        [Fact]
+        public void BasicFunctionTest()
+        {
+            const string input = @"
+function testFunc ()
+    a = 100
+end
+a = 50
+testFunc()
+";
+            var script = new Script(input);
+            Assert.Empty(script.Diagnostics.Messages);
+            script.Evaluate();
+            Assert.Empty(script.Diagnostics.Messages);
+            Assert.True(script.Scope.TryGet("testFunc", out var func));
+            Assert.IsType<LuaFunction>(func);
+            Assert.True(script.Scope.TryGet("a", out var a));
+            Assert.Equal(100, (long)(NumberLong)a);
+        }
+    }
+}