CLI: let --export-settings - write the merged config to stdout

--export-settings already writes the merged config as JSON at the right
point in the CLI flow. Passing - writes the same document to stdout.

- ConfigBase::save_to_json gains a stream overload. The file overload
  serializes through it before opening the file, so the format is
  unchanged and a config that cannot be serialized leaves an existing
  file untouched instead of truncating it.
- On stdout, invalid UTF-8 in string values is written as U+FFFD instead
  of ending the process with an uncaught type_error; files keep the
  strict behaviour.
- - is rejected up front when combined with an action or transform that
  can write to stdout or does real work, so stdout carries only the
  JSON.
- The unconditional "skip locked instance" stdout write during arrange
  now goes to the log.
- Tests in tests/libslic3r/test_config.cpp.

src/libslic3r/PrintConfig.cpp

@@ -11916,7 +11916,7 @@ CLIActionsConfigDef::CLIActionsConfigDef()
 
     def = this->add("export_settings", coString);
     def->label = L("Export Settings");
-    def->tooltip = L("This exports settings to a file.");
+    def->tooltip = L("This exports settings to a file. Use - to write them to stdout.");
     def->cli_params = "settings.json";
     def->set_default_value(new ConfigOptionString("output.json"));