implement support for multiple index operations

examples/index.pork

@@ -0,0 +1,4 @@
+export func main() {
+  let items = [["Hello"], ["Goodbye"]]
+  println(items[0][0])
+}

parser/src/main/kotlin/gay/pizza/pork/parser/Parser.kt

@@ -63,7 +63,7 @@ class Parser(source: TokenSource, attribution: NodeAttribution) :
       }
     }
 
-    if (peek(TokenType.LeftBracket)) {
+    while (peek(TokenType.LeftBracket)) {
       expression = produce(NodeType.IndexedBy) {
         attribution.adopt(expression)
         expect(TokenType.LeftBracket)