Fix nim compilation in case a edge case of a schema with enum (#20780)

If enum constraint contains one value which is not a valid nim
identifier, we must surround this identifier with backtick.

Author: sandwoodK

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/NimClientCodegen.java

@@ -355,15 +355,21 @@ public String toEnumName(CodegenProperty property) {
         }
     }
 
+    private boolean isValidIdentifier(String identifier) {
+        //see https://nim-lang.org/docs/manual.html#lexical-analysis-identifiers-amp-keywords
+        return identifier.matches("^(?:[A-Z]|[a-z]|[\\x80-\\xff])(_?(?:[A-Z]|[a-z]|[\\x80-\\xff]|[0-9]))*$");
+    }
+
     @Override
     public String toEnumVarName(String name, String datatype) {
         name = name.replace(" ", "_");
         name = StringUtils.camelize(name);
 
-        if (name.matches("\\d.*")) { // starts with number
-            return "`" + name + "`";
-        } else {
+        // starts with number or contains any character not allowed,see
+        if (isValidIdentifier(name)) {
             return name;
+        } else {
+            return "`" + name + "`";
         }
     }