Fix multiline string stability (#1287)

pkl-formatter/src/main/kotlin/org/pkl/formatter/Generator.kt

@@ -89,6 +89,7 @@ internal class Generator {
       }
       is MultilineStringGroup -> {
         val indentLength = indent * INDENT.length
+        val oldIndent = indentFor(node)
         var previousNewline = false
         for ((i, child) in node.nodes.withIndex()) {
           when {
@@ -96,7 +97,7 @@ internal class Generator {
             child is Text &&
               previousNewline &&
               child.text.isBlank() &&
-              child.text.length == indentLength &&
+              child.text.length == oldIndent &&
               node.nodes[i + 1] is ForceLine -> {}
             child is Text && previousNewline ->
               text(reposition(child.text, node.endQuoteCol - 1, indentLength))
@@ -128,6 +129,15 @@ internal class Generator {
   private fun reposition(text: String, originalOffset: Int, newOffset: Int): String =
     " ".repeat(newOffset) + text.drop(originalOffset)
 
+  // Returns the indent of this multiline string, which is the size of the last node before the
+  // closing quotes, or 0 if the closing quotes have no indentation
+  private fun indentFor(multi: MultilineStringGroup): Int {
+    val nodes = multi.nodes
+    if (nodes.size < 2) return 0
+    val beforeLast = nodes[nodes.lastIndex - 1]
+    return if (beforeLast is Text) beforeLast.text.length else 0
+  }
+
   override fun toString(): String {
     return buf.toString()
   }

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

@@ -47,3 +47,12 @@ quux {
 
 		"""
 }
+
+obj {
+    data {
+      ["bar"] = """
+        foo
+        
+        """
+    }
+}

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

@@ -51,3 +51,13 @@ quux {
 
   """
 }
+
+obj {
+  data {
+    ["bar"] =
+      """
+      foo
+
+      """
+  }
+}