[ruby/yarp] fix: %W list spanning a heredoc

Primarily this fix is to accept a string node and concatenate it onto
an interpolated string.

https://github.com/ruby/yarp/commit/6df729fe72

test/yarp/fixtures/spanning_heredoc.txt

@@ -29,3 +29,10 @@ pp <<-A, %w[j\
 i
 A
 j]
+
+# ripper can't parse this successfully, though ruby runs it correctly
+# TODO: yarp does not include the "\n" in "l\nl" in the AST like ruby does
+pp <<-A, %W[l\
+k
+A
+l]

test/yarp/snapshots/spanning_heredoc.txt

@@ -1,6 +1,6 @@
-ProgramNode(164...541)(
+ProgramNode(164...709)(
   [],
-  StatementsNode(164...541)(
+  StatementsNode(164...709)(
     [CallNode(164...192)(
        nil,
        nil,
@@ -133,6 +133,33 @@ ProgramNode(164...541)(
        nil,
        0,
        "pp"
+     ),
+     CallNode(688...709)(
+       nil,
+       nil,
+       (688...690),
+       nil,
+       ArgumentsNode(691...709)(
+         [InterpolatedStringNode(691...695)(
+            (691...695),
+            [StringNode(703...705)(nil, (703...705), nil, "k\n")],
+            (705...707)
+          ),
+          ArrayNode(697...709)(
+            [InterpolatedStringNode(700...708)(
+               nil,
+               [StringNode(700...703)(nil, (700...703), nil, "l"),
+                StringNode(707...708)(nil, (707...708), nil, "l")],
+               nil
+             )],
+            (697...700),
+            (708...709)
+          )]
+       ),
+       nil,
+       nil,
+       0,
+       "pp"
      )]
   )
 )

yarp/yarp.c

@@ -12160,6 +12160,19 @@ parse_expression_prefix(yp_parser_t *parser, yp_binding_power_t binding_power) {
                             // to the list of child nodes.
                             yp_node_t *part = parse_string_part(parser);
                             yp_interpolated_string_node_append((yp_interpolated_string_node_t *) current, part);
+                        } else if (YP_NODE_TYPE_P(current, YP_NODE_STRING_NODE)) {
+                            // If we hit string content and the current node is a string node,
+                            // then we need to convert the current node into an interpolated
+                            // string and add the string content to the list of child nodes.
+                            yp_token_t opening = not_provided(parser);
+                            yp_token_t closing = not_provided(parser);
+                            yp_interpolated_string_node_t *interpolated =
+                                yp_interpolated_string_node_create(parser, &opening, NULL, &closing);
+                            yp_interpolated_string_node_append(interpolated, current);
+
+                            yp_node_t *part = parse_string_part(parser);
+                            yp_interpolated_string_node_append(interpolated, part);
+                            current = (yp_node_t *) interpolated;
                         } else {
                             assert(false && "unreachable");
                         }