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/Config.cpp

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <assert.h>
 #include <fstream>
+#include <sstream>
 #include <iostream>
 #include <iomanip>
 #include <regex>
@@ -1515,6 +1516,19 @@ std::optional<PluginCapabilityRef> parse_capability_ref(const std::string& value
 
 //BBS: add json support
 void ConfigBase::save_to_json(const std::string &file, const std::string &name, const std::string &from, const std::string &version) const
+{
+    // Serialize first: if that throws (invalid UTF-8), the existing file stays untouched.
+    std::ostringstream ss;
+    this->save_to_json(ss, name, from, version);
+    boost::nowide::ofstream c;
+    c.open(file, std::ios::out | std::ios::trunc);
+    c << ss.str();
+    c.close();
+
+    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ":" <<__LINE__ << boost::format(", saved config to %1%\n")%file;
+}
+
+void ConfigBase::save_to_json(std::ostream &os, const std::string &name, const std::string &from, const std::string &version, bool replace_invalid_utf8) const
 {
     json j;
     //record the headers
@@ -1561,12 +1575,7 @@ void ConfigBase::save_to_json(const std::string &file, const std::string &name,
             j["plugins"] = unique_refs;
     }
 
-    boost::nowide::ofstream c;
-    c.open(file, std::ios::out | std::ios::trunc);
-    c << j.dump(1, '\t') << std::endl;
-    c.close();
-
-    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ":" <<__LINE__ << boost::format(", saved config to %1%\n")%file;
+    os << j.dump(1, '\t', false, replace_invalid_utf8 ? json::error_handler_t::replace : json::error_handler_t::strict) << std::endl;
 }
 
 void ConfigBase::save(const std::string &file) const