Add undefined and null literal support

This adds the UndefinedLiteral and NullLiteral to AST.

Fixes #990

docs/compiler-api.md

@@ -164,6 +164,14 @@ interface NumberLiteral <: Literal {
     value: number;
     original: number;
 }
+
+interface UndefinedLiteral <: Literal {
+    type: "UndefinedLiteral";
+}
+
+interface NullLiteral <: Literal {
+    type: "NullLiteral";
+}
 ```