SPICE-0028: Add support for multi-line string line continuations (#1507)

SPICE: https://github.com/apple/pkl-evolution/pull/31

pkl-formatter/src/main/java/org/pkl/formatter/Builder.java

@@ -75,6 +75,11 @@ final class Builder {
           OPERATOR ->
           new Text(node.text(source));
       case STRING_NEWLINE -> mustForceLine();
+      case STRING_CONTINUATION -> {
+        var escape = node.text(source);
+        yield new Nodes(
+            List.of(new Text(escape.substring(0, escape.length() - 1)), mustForceLine()));
+      }
       case MODULE_DECLARATION -> formatModuleDeclaration(node);
       case MODULE_DEFINITION -> formatModuleDefinition(node);
       case SINGLE_LINE_STRING_LITERAL_EXPR -> formatSingleLineString(node);
@@ -922,7 +927,9 @@ final class Builder {
       if (elem.type == NodeType.TERMINAL && text(elem).endsWith("(")) {
         isInStringInterpolation = true;
       }
-      result.add(format(elem));
+      var formatted = format(elem);
+      if (formatted instanceof Nodes formattedNodes) result.addAll(formattedNodes.nodes());
+      else result.add(formatted);
       prev = elem;
     }
     return result;

pkl-formatter/src/test/files/FormatterSnippetTests/input/multi-line-strings.pkl

@@ -48,6 +48,26 @@ quux {
 		"""
 }
 
+// line continuations
+corge {
+
+  """
+  hello \
+  world
+  """
+
+#"""
+hello \#
+world
+"""#
+
+"""
+hello \
+\
+world
+"""
+}
+
 obj {
     data {
       ["bar"] = """

pkl-formatter/src/test/files/FormatterSnippetTests/output/multi-line-strings.pkl

@@ -52,6 +52,25 @@ quux {
   """
 }
 
+// line continuations
+corge {
+  """
+  hello \
+  world
+  """
+
+  #"""
+  hello \#
+  world
+  """#
+
+  """
+  hello \
+  \
+  world
+  """
+}
+
 obj {
   data {
     ["bar"] =