Implement optional trailing commas (#97)

tests/parser/trailing-commas.ts

@@ -0,0 +1,33 @@
+enum Foo {
+  A,
+  B,
+}
+
+function add(
+  x: i32,
+  y: i32,
+): i32 {
+  return x + y;
+}
+
+function parameterized<
+  A,
+  B,
+>(a: A, b: B): void {
+}
+
+export function compute(): i32 {
+  const arr: Array<i8> = [
+    1,
+    2,
+  ];
+  parameterized<
+    i8,
+    // @ts-ignore: Waiting on https://github.com/Microsoft/TypeScript/issues/21984
+    i32,
+  >(0, 0);
+  return add(
+    1,
+    2,
+  );
+}