highlights(hack): add keywords implements

tests/query/highlights/hack/attribute-type.hack

@@ -9,7 +9,7 @@ newtype T1 = ?shape(
 //       ^ attribute
 type T2 = (function(T1): string);
 //    ^ type
-//          ^ function (cannot capture keyword "function" as keyword.function)
+//          TODO: keyword.function (currently not in AST)
 
 <<A4(1), A5, A6(1,3,4)>>
 newtype T3 as int = int;

tests/query/highlights/hack/generics.hack

@@ -8,6 +8,7 @@ class Box<T> {
   public function __construct(T $data) {
   //                          ^ type
   //                             ^ parameter
+  //        ^ keyword.function
   // ^ keyword
   //                    ^ method
     $this->data = $data;

tests/query/highlights/hack/shapes.hack

@@ -0,0 +1,12 @@
+class C extends Superclass implements Iface {
+//      ^ keyword          ^ keyword
+  use Trait;
+  // < include
+  const type X = shape(
+  // <- keyword ^ type.builtin
+    "a" => int,
+  // ^ string
+    "b" => string,
+  //       ^ type.builtin
+  );
+}