migrate from mocha to jest; warn on unexpected siblings for _content

.babelrc

@@ -0,0 +1,7 @@
+{
+    "env": {
+        "test": {
+            "plugins": ["@babel/plugin-transform-modules-commonjs"]
+        }
+    }
+}

.eslintrc

@@ -6,6 +6,6 @@
     "env": {
         "browser": true,
         "node": true,
-        "mocha": true
+        "test": true
     }
 }

eslint.config.mjs

@@ -22,8 +22,7 @@ export default defineConfig([
         languageOptions: {
             globals: {
                 ...globals.browser,
-                ...globals.node,
-                ...globals.mocha
+                ...globals.node
             },
 
             parser: babelParser,

jstoxml.js

@@ -222,7 +222,13 @@ export const toXML = (obj = {}, config = {}) => {
         case DATA_TYPES.JSTOXML_OBJECT: {
             // Processes a specially-formatted object used by jstoxml.
 
-            const { _name, _content } = obj;
+            const { _name, _content, _attrs, ...invalidSiblings } = obj;
+
+            if (Object.keys(invalidSiblings).length > 0) {
+                console.warn(
+                    `Unexpected sibling ${Object.keys(invalidSiblings).join(', ')}.  When using _content, the only siblings recognized are _name and _attrs.`
+                );
+            }
 
             // Output text content without a tag wrapper.
             if (_content === null && typeof contentMap !== 'function') {