Add lowercase and uppercase string functions, update documentation accordingly.

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjString.kt

@@ -216,6 +216,14 @@ data class ObjString(val value: String) : Obj() {
             ) {
                 thisAs<ObjString>().value.lowercase().let(::ObjString)
             }
+            addFnDoc(
+                name = "lowercase",
+                doc = "Lowercase version of this string (default locale).",
+                returns = type("lyng.String"),
+                moduleName = "lyng.stdlib"
+            ) {
+                thisAs<ObjString>().value.lowercase().let(::ObjString)
+            }
             addFnDoc(
                 name = "upper",
                 doc = "Uppercase version of this string (default locale).",
@@ -224,6 +232,14 @@ data class ObjString(val value: String) : Obj() {
             ) {
                 thisAs<ObjString>().value.uppercase().let(::ObjString)
             }
+            addFnDoc(
+                name = "uppercase",
+                doc = "Uppercase version of this string (default locale).",
+                returns = type("lyng.String"),
+                moduleName = "lyng.stdlib"
+            ) {
+                thisAs<ObjString>().value.uppercase().let(::ObjString)
+            }
             addFnDoc(
                 name = "characters",
                 doc = "List of characters of this string.",