Fix variable resolution of object body params (#1788)

Fixes a regression introduced in Pkl 0.32.

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorObjectBodyParam.pkl

@@ -0,0 +1,9 @@
+foo = new Mapping {
+  default { key ->
+    for (idx, char in key.split("")) {
+      ["\(char)_\(idx)"] = "hi"
+    }
+  }
+  ["zzz"] {}
+}
+

pkl-core/src/test/files/LanguageSnippetTests/input/generators/whenGeneratorObjectBodyParam.pkl

@@ -0,0 +1,8 @@
+foo = new Mapping {
+  default { key ->
+    when (key is String) {
+      bar = key
+    }
+  }
+  ["x"] {}
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorObjectBodyParam.pcf

@@ -0,0 +1,7 @@
+foo {
+  ["zzz"] {
+    ["z_0"] = "hi"
+    ["z_1"] = "hi"
+    ["z_2"] = "hi"
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/whenGeneratorObjectBodyParam.pcf

@@ -0,0 +1,5 @@
+foo {
+  ["x"] {
+    bar = "x"
+  }
+}