YaML: escape exclamation marks to avoid issues with compliant parsers

and added some support for reading such escaped characters.

radio/src/storage/yaml/yaml_datastructs_funcs.cpp

@@ -763,7 +763,7 @@ static bool w_swtchSrc(const YamlNode* node, uint32_t val, yaml_writer_func wf,
 {
     int32_t sval = yaml_to_signed(val, node->size);
     if (sval < 0) {
-        wf(opaque, "!", 1);
+        wf(opaque, "\\!", 2);
         sval = abs(sval);
     }